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  • AMIYA

    GEOMETRY CONCEPTS &

    THEOREMS

    3 E L E A R N I N G , 3 R D F L O O RH . B . R O A D

    GEOMETRY CONCEPTS &

    REMS COMPILATION MATHS BY AMIYA

    Amiya

    L O O R , A N A N D C O M P L E X , N E A R O A D R A N C H I , 0 9 5 3 4 0 0 2 2 4 4

    E A R L A L P U R P S , 0 0 2 2 4 4

  • 3E Learning, 3rd Floor, Anand Complex, Near Lalpur PS, H.B. Road Ranchi, 095 34 002244www.facebook.com/MathsByAmiya

    Barbier's theorem

    In geometry, Barbier's theorem states that every curve of constant width

    regardless of its precise shape.

    Curve of constant width :- In geometry, a curve

    (defined as the perpendicular distance between two distinct parallel lines each having at least one point in

    common with the shape's boundary but none with the shape's interior) is the same regardless

    orientation of the curve.

    More generally, any compact convex planar body D has one pair of parallel supporting lines in any given

    direction. A supporting line is a line that has at least one point in common with the boundary of D but no

    points in common with the interior of D. The width of the body is defined as before. If the width of D is the

    same in all directions, the body is said to have constant width and its boundary is a curve of constant width;

    the planar body itself is called an orbiform.

    The width of a circle is constant: its diameter. On the other hand, the width of a square varies between the

    length of a side and that of a diagonal, in the ratio

    constant in all directions, is it necessarily a circle? The surprising answer is that there are many non

    circular shapes of constant width. A nontrivial example is the Reuleaux triangle. To construct this, take an

    equilateral triangle with vertices ABC and draw the arc BC on the circle cen

    circle centered at B, and the arc AB on the circle centered at C. The resulting figure is of constant width.

    The Reuleaux triangle lacks tangent continuity at three points, but constant

    constructed without such discontinuities (as shown in the second illustration on the right). Curves of

    constant width can be generated by joining circular arcs centered on the vertices of a regular or irregular

    convex polygon with an odd number of sides (triangle, penta

    The most familiar examples of curves of constant width are the circle

    and the Reuleaux triangle. For a circle, the width is the same as the

    diameter; a circle of width w has perimeter w. A Reuleaux triangle

    of width w consists of three arcs of circles of radius w. Each of these

    arcs has central angle /3, so the perimeter of the Reuleaux triangle

    of width w is equal to half the perimeter of a circle of radius w and

    therefore is equal to w. A similar analysis of other simple exampl

    such as Reuleaux polygons gives the same answer.

    The analogue of Barbier's theorem for

    false. In particular, the unit sphere has surface area

    while the surface of revolution of a Reuleaux triangle

    constant width has surface area

    3E Learning, 3rd Floor, Anand Complex, Near Lalpur PS, H.B. Road Ranchi, 095 34 002244www.facebook.com/MathsByAmiya AMIYA KUMAR

    These Reuleaux polygons have

    constant width, and all have the

    same width; therefore by

    Barbier's theorem they also have

    equal perimeters.

    In geometry, Barbier's theorem states that every curve of constant width* has perimeter times its width,

    In geometry, a curve of constant width is a convex planar shape whose width

    (defined as the perpendicular distance between two distinct parallel lines each having at least one point in

    common with the shape's boundary but none with the shape's interior) is the same regardless

    More generally, any compact convex planar body D has one pair of parallel supporting lines in any given

    direction. A supporting line is a line that has at least one point in common with the boundary of D but no

    common with the interior of D. The width of the body is defined as before. If the width of D is the

    same in all directions, the body is said to have constant width and its boundary is a curve of constant width;

    the planar body itself is called an orbiform.

    The width of a circle is constant: its diameter. On the other hand, the width of a square varies between the

    length of a side and that of a diagonal, in the ratio

    . Thus the question arises: if a given shape's width is

    necessarily a circle? The surprising answer is that there are many non

    circular shapes of constant width. A nontrivial example is the Reuleaux triangle. To construct this, take an

    equilateral triangle with vertices ABC and draw the arc BC on the circle centered at A, the arc CA on the

    circle centered at B, and the arc AB on the circle centered at C. The resulting figure is of constant width.

    The Reuleaux triangle lacks tangent continuity at three points, but constant-width curves can also be

    thout such discontinuities (as shown in the second illustration on the right). Curves of

    constant width can be generated by joining circular arcs centered on the vertices of a regular or irregular

    convex polygon with an odd number of sides (triangle, pentagon, heptagon, etc.).

    The most familiar examples of curves of constant width are the circle

    and the Reuleaux triangle. For a circle, the width is the same as the

    diameter; a circle of width w has perimeter w. A Reuleaux triangle

    three arcs of circles of radius w. Each of these

    arcs has central angle /3, so the perimeter of the Reuleaux triangle

    of width w is equal to half the perimeter of a circle of radius w and

    therefore is equal to w. A similar analysis of other simple examples

    such as Reuleaux polygons gives the same answer.

    The analogue of Barbier's theorem for surfaces of constant width is

    has surface area ,

    Reuleaux triangle with the same

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    These Reuleaux polygons have

    constant width, and all have the

    same width; therefore by

    Barbier's theorem they also have

    equal perimeters.

    has perimeter times its width,

    of constant width is a convex planar shape whose width

    (defined as the perpendicular distance between two distinct parallel lines each having at least one point in

    common with the shape's boundary but none with the shape's interior) is the same regardless of the

    More generally, any compact convex planar body D has one pair of parallel supporting lines in any given

    direction. A supporting line is a line that has at least one point in common with the boundary of D but no

    common with the interior of D. The width of the body is defined as before. If the width of D is the

    same in all directions, the body is said to have constant width and its boundary is a curve of constant width;

    The width of a circle is constant: its diameter. On the other hand, the width of a square varies between the

    . Thus the question arises: if a given shape's width is

    necessarily a circle? The surprising answer is that there are many non-

    circular shapes of constant width. A nontrivial example is the Reuleaux triangle. To construct this, take an

    tered at A, the arc CA on the

    circle centered at B, and the arc AB on the circle centered at C. The resulting figure is of constant width.

    width curves can also be

    thout such discontinuities (as shown in the second illustration on the right). Curves of

    constant width can be generated by joining circular arcs centered on the vertices of a regular or irregular

    gon, heptagon, etc.).

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    Brahmagupta theorem

    In geometry, Brahmagupta's theorem

    has perpendicular diagonals), then the perpendicular to a side from the point of intersection of the

    diagonals always bisects the opposite side.

    More specifically, let A, B, C and D

    lines AC and BD are perpendicular. Denote the intersection of

    AC and BD by M. Drop the perpendicular from

    the intersection E. Let F be the intersection of the line

    edge AD. Then, the theorem states that

    Proof :-

    We need to prove that AF = FD. We will prove that both

    To prove that AF = FM, first note that the angles

    because they are inscribed angles that intercept the same arc of the circle.

    Furthermore, the angles CBM and CME

    angle BCM (i.e., they add up to 90), and are therefore equal. Finally,

    the angles CME and FMA are the same. Hence,

    triangle, and thus the sides AF and FM

    The proof that FD = FM goes similarly: the

    angles FDM, BCM, BME and DMF

    triangle, so FD = FM. It follows that

    Brahmagupta's formula

    Brahmagupta's formula gives the area

    where s, the semiperimeter, is defined to be

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    Brahmagupta theorem

    Brahmagupta's theorem states that if a cyclic quadrilateral is orthodiagonal

    diagonals), then the perpendicular to a side from the point of intersection of the

    the opposite side.

    be four points on a circle such that the

    are perpendicular. Denote the intersection of

    . Drop the perpendicular from M to the line BC, calling

    be the intersection of the line EM and the

    e theorem states that F is the midpoint AD.

    . We will prove that both AF and FD are in fact equal to

    , first note that the angles FAM and CBM are equal,

    that intercept the same arc of the circle.

    CME are both complementary to

    (i.e., they add up to 90), and are therefore equal. Finally,

    are the same. Hence, AFM is an isosceles

    FM are equal.

    goes similarly: the

    DMF are all equal, so DFM is an isosceles

    . It follows that AF = FD, as the theorem claims.

    Brahmagupta's formula

    Brahmagupta's formula gives the area A of a cyclic quadrilateral whose sides have lengths

    semiperimeter, is defined to be

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    orthodiagonal (that is,

    diagonals), then the perpendicular to a side from the point of intersection of the

    are in fact equal to FM.

    whose sides have lengths a, b, c, d as

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    This formula generalizes Heron's formula

    regarded as a quadrilateral with one side of length zero. From this perspective, as

    approaches zero, a cyclic quadrilateral converges into a cyclic triangle (all triangles are

    cyclic), and Brahmagupta's formula simplifies to Heron's formula.

    If the semiperimeter is not used, Brahmagupta's formula is

    Another equivalent version is

    Extension to non-cyclic quadrilaterals

    In the case of non-cyclic quadrilaterals, Brahmagupta's formula can be extended by considering the

    measures of two opposite angles of the

    where is half the sum of two opposite angles. (The choice of which pair of opposite angles is irrelevant: if

    the other two angles are taken, half their

    cos2(180 ) = cos

    2.) This more general formula is known as

    It is a property of cyclic quadrilaterals

    quadrilateral sum to 180. Consequently, in the case of an inscribed quadrilater

    It follows from the latter equation that the area of a cyclic quadrilateral is the maximum possible area for

    any quadrilateral with the given side lengths.

    A related formula, which was proved by

    is

    where p and q are the lengths of the diagonals of the quadrilateral. In a

    according to Ptolemy's theorem, and the formula of Coolidge reduces

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    Heron's formula for the area of a triangle. A triangle may be

    a quadrilateral with one side of length zero. From this perspective, as

    approaches zero, a cyclic quadrilateral converges into a cyclic triangle (all triangles are

    cyclic), and Brahmagupta's formula simplifies to Heron's formula.

    not used, Brahmagupta's formula is

    cyclic quadrilaterals

    cyclic quadrilaterals, Brahmagupta's formula can be extended by considering the

    measures of two opposite angles of the quadrilateral:

    the sum of two opposite angles. (The choice of which pair of opposite angles is irrelevant: if

    their sum is the supplement of . Since cos(180

    .) This more general formula is known as Bretschneider's formula

    cyclic quadrilaterals (and ultimately of inscribed angles) that opposite angles of a

    quadrilateral sum to 180. Consequently, in the case of an inscribed quadrilateral, = 90, whence the term

    giving the basic form of Brahmagupta's formula.

    It follows from the latter equation that the area of a cyclic quadrilateral is the maximum possible area for

    any quadrilateral with the given side lengths.

    hich was proved by Coolidge, also gives the area of a general convex quadrilateral. It

    are the lengths of the diagonals of the quadrilateral. In a cyclic uadrilateral

    , and the formula of Coolidge reduces to Brahmagupta's formula.

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    . A triangle may be

    a quadrilateral with one side of length zero. From this perspective, as d

    approaches zero, a cyclic quadrilateral converges into a cyclic triangle (all triangles are

    cyclic quadrilaterals, Brahmagupta's formula can be extended by considering the

    the sum of two opposite angles. (The choice of which pair of opposite angles is irrelevant: if

    sum is the supplement of . Since cos(180 ) = cos, we have

    Bretschneider's formula.

    ) that opposite angles of a

    al, = 90, whence the term

    giving the basic form of Brahmagupta's formula.

    It follows from the latter equation that the area of a cyclic quadrilateral is the maximum possible area for

    , also gives the area of a general convex quadrilateral. It

    cyclic uadrilateral,

    to Brahmagupta's formula.

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    Butterfly theorem

    The butterfly theorem is a classical result in

    which can be stated as follows:

    Let M be the midpoint of a chord

    which two other chords AB and

    drawn; AD and BC intersect chord

    at X and Y correspondingly. Then

    Proof:-

    Let the perpendiculars and

    and respectively. Similarly, let

    from the point perpendicular to the straight lines

    respectively.

    Now, since

    From the preceding equations, it can be easily seen that

    since =

    Now,

    So, it can be concluded that

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    is a classical result in Euclidean geometry,

    chord PQ of a circle, through

    and CD are

    chord PQ

    correspondingly. Then M is the midpoint of XY.

    be dropped from the point on the straight lines

    respectively. Similarly, let and be dropped

    perpendicular to the straight lines and

    From the preceding equations, it can be easily seen that

    or is the midpoint of

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    on the straight lines

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    Casey's theorem

    In mathematics, Casey's theorem, also known as the

    generalized Ptolemy's theorem, is a theorem in

    geometry named after the Irish mathematician

    Let be a circle of radius . Let

    that order) four non-intersecting circles that

    tangent to it. Denote by the length of the exterior common

    tangent of the circles .

    Then:

    Note that in the degenerate case, where all four circles reduce

    to points, this is exactly Ptolemy's theorem

    De Gua's theorem

    De Gua's theorem is a three-dimensional analog of the

    Gua de Malves.

    If a tetrahedron has a right-angle corner (like the corner of a

    square of the area of the face opposite the right

    the squares of the areas of the other three faces.

    The Pythagorean theorem and de Gua's theorem are special cases (

    a general theorem about n-simplices

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    , also known as the

    Ptolemy's theorem, is a theorem in Euclidean

    mathematician John Casey.

    be (in

    intersecting circles that lie inside and

    the length of the exterior common

    Note that in the degenerate case, where all four circles reduce

    Ptolemy's theorem.

    dimensional analog of the Pythagorean theorem and named for

    angle corner (like the corner of a cube), then the

    square of the area of the face opposite the right-angle corner is the sum of

    the squares of the areas of the other three faces.

    and de Gua's theorem are special cases (n = 2, 3) of

    simplices with a right angle corner.

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    and named for Jean Paul de

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    Descartes' theorem

    In geometry, Descartes' theorem states that for every four

    the circles satisfy a certain quadratic equation. By solving this equation, one can construct a fourth circle

    tangent to three given, mutually tangent circles. The theor

    in 1643.

    Definition of curvature

    Kissing circles. Given three mutually tangent circles (

    fourth tangent circle have? There are in general two possible answers (

    Descartes' theorem is most easily stated in terms of the circles'

    The curvature (or bend) of a circle is defined as

    radius. The larger a circle, the smaller is the

    and vice versa.

    The plus sign in k = 1/r applies to a circle that is

    other circles, like the three black circles in the image. For an

    internally tangent circle like the big red circle, that

    circles, the minus sign applies.

    If a straight line is considered a degenerate

    theorem also applies to a line and two circles that are all three mutually tangent, giving the radius of a third

    circle tangent to the other two circles and the line.

    If four circles are tangent to each other at six distinct points, and the circles have

    curvatures ki (for i = 1, ..., 4), Descartes' theorem says:

    When trying to find the radius of a fourth circle tangent to three given

    rewritten as:

    The sign reflects the fact that there are in general

    line, one solution is positive and the other is either

    negative; if negative, it represents a circle that circumscribes the

    first three (as shown in the diagram above).

    favor one solution over the other in any given problem.

    SPECIAL CASE

    If one of the three circles is replaced

    one ki, say k3, is zero and drops out of

    (2) then becomes much simpler:

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    Descartes' theorem

    states that for every four kissing, or mutually tangent,

    quadratic equation. By solving this equation, one can construct a fourth circle

    tangent to three given, mutually tangent circles. The theorem is named after Ren Descartes, who stated it

    Kissing circles. Given three mutually tangent circles (black), what radius can a

    fourth tangent circle have? There are in general two possible answers (red).

    rem is most easily stated in terms of the circles' curvatures.

    ) of a circle is defined as k = 1/r, where r is its

    radius. The larger a circle, the smaller is the magnitude of its curvature,

    applies to a circle that is externally tangent to the

    other circles, like the three black circles in the image. For an

    tangent circle like the big red circle, that circumscribes the other

    degenerate circle with zero curvature (and thus infinite radius), Descartes'

    theorem also applies to a line and two circles that are all three mutually tangent, giving the radius of a third

    circle tangent to the other two circles and the line.

    If four circles are tangent to each other at six distinct points, and the circles have

    4), Descartes' theorem says:

    ..........................

    When trying to find the radius of a fourth circle tangent to three given kissing circles, the equation is best

    ..........................

    The sign reflects the fact that there are in general two solutions. Ignoring the degenerate case of a straight

    line, one solution is positive and the other is either positive or

    negative; if negative, it represents a circle that circumscribes the

    first three (as shown in the diagram above). Other criteria may

    favor one solution over the other in any given problem.

    If one of the three circles is replaced by a straight line, then

    , is zero and drops out of equation (1). Equation

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    tangent, circles, the radii of

    quadratic equation. By solving this equation, one can construct a fourth circle

    Ren Descartes, who stated it

    circle with zero curvature (and thus infinite radius), Descartes'

    theorem also applies to a line and two circles that are all three mutually tangent, giving the radius of a third

    If four circles are tangent to each other at six distinct points, and the circles have

    ..........................(1)

    kissing circles, the equation is best

    ..........................(2)

    solutions. Ignoring the degenerate case of a straight

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    Japanese theorem for cyclic polygons

    In geometry, the Japanese theorem

    the sum of inradii of triangles is constant.

    sum of the radii of the green circles = sum of the radii of the red circles

    Conversely, if the sum of inradii independent from the triangulation, then the polygon is cyclic.

    Carnot's theorem

    In Euclidean geometry, Carnot's theorem

    after Lazare Crarnot (17531823), is as follows. Let

    an arbitrary triangle. Then the sum of the

    distances from the circumcenter D to the sides of

    triangle ABC is

    where r is the inradius and R is the circumradius. Here the

    sign of the distances is taken negative if and only if the

    segment DX (X = F, G, H) lies completely outside the

    triangle. In the picture DF is negative and

    both DG and DH are positive.

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    Japanese theorem for cyclic polygons

    Japanese theorem states that no matter how one triangulates a cyclic

    constant.

    sum of the radii of the green circles = sum of the radii of the red circles

    Conversely, if the sum of inradii independent from the triangulation, then the polygon is cyclic.

    Carnot's theorem, named

    1823), is as follows. Let ABC be

    triangle. Then the sum of the signed

    to the sides of

    circumradius. Here the

    is taken negative if and only if the line

    ) lies completely outside the

    is negative and

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    cyclic polygon,

    Conversely, if the sum of inradii independent from the triangulation, then the polygon is cyclic.

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    Pitot theorem

    A tangential quadrilateral ABCD is a closed figure of four straight

    sides that are tangent to a given circle

    inscribed in the quadrilateral ABCD.

    This conclusion follows from the equality of the tangent segments

    from the four vertices of the quadrilateral. Let the tangent points be

    denoted as P (on segment AB), Q (on segment BC),

    CD) and S (on segment DA). The symmetric tangent segments about

    each point of ABCD are equal, e.g., BP=BQ=

    DR=DS=d, and AS=AP=a. But each side of the quadrilateral is composed of two such tangent segments

    The converse is also true: a circle can be inscribed into every quadrilateral in which the lengths of opposite

    sides sum to the same value.

    This theorem and its converse have various uses. For example, they show immediately that no rectangle

    can have an inscribed circle unless it is a

    a general parallelogram does not.

    Japanese theorem for cyclic quadrilaterals

    In geometry, the Japanese theorem

    the incircles of certain triangles inside a

    quadrilateral are vertices of a rectangle.

    Triangulating an arbitrary concyclic quadrilateral by its

    diagonals yields four overlapping triangle

    diagonal creates two triangles). The centers of the

    incircles of those triangles form a rectangle.

    Specifically, let be an arbitrary concyclic

    quadrilateral and let be

    incenters of the triangles

    Then the quadrilateral formed by

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    ABCD is a closed figure of four straight

    sides that are tangent to a given circle C. Equivalently, the circle C is

    in the quadrilateral ABCD.

    This conclusion follows from the equality of the tangent segments

    ces of the quadrilateral. Let the tangent points be

    (on segment BC), R (on segment

    (on segment DA). The symmetric tangent segments about

    each point of ABCD are equal, e.g., BP=BQ=b, CQ=CR=c,

    But each side of the quadrilateral is composed of two such tangent segments

    a circle can be inscribed into every quadrilateral in which the lengths of opposite

    various uses. For example, they show immediately that no rectangle

    can have an inscribed circle unless it is a square, and that every rhombus has an inscribed circle, whereas

    Japanese theorem for cyclic quadrilaterals

    Japanese theorem states that the centers of

    inside a cyclic

    are vertices of a rectangle.

    Triangulating an arbitrary concyclic quadrilateral by its

    diagonals yields four overlapping triangles (each

    diagonal creates two triangles). The centers of the

    incircles of those triangles form a rectangle.

    be an arbitrary concyclic

    the

    .

    is a rectangle.

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    But each side of the quadrilateral is composed of two such tangent segments

    a circle can be inscribed into every quadrilateral in which the lengths of opposite

    various uses. For example, they show immediately that no rectangle

    square, and that every rhombus has an inscribed circle, whereas

    Japanese theorem for cyclic quadrilaterals

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    Pick's theorem

    Given a simple polygon constructed on a grid of equal

    with integer coordinates) such that all the polygon's vertices are grid points,

    simple formula for calculating the area

    interiorlocated in the polygon and the number

    boundary placed on the polygon's perimeter

    In the example shown, we have i = 7 interior points and

    points, so the area is A = 7 + 8/2 1 =

    Note that the theorem as stated above is only valid for

    i.e., ones that consist of a single piece and do not contain "holes". For a polygon that has

    boundary in the form of h + 1 simple closed curves, the slightly more complicated formula

    gives the area.

    Ptolemy's theorem

    In Euclidean geometry, Ptolemy's theorem

    a cyclic quadrilateral (a quadrilateral whose

    The theorem is named after

    the Greek astronomer and mathematician

    Ptolemaeus) Ptolemy used the theorem as an aid to creating

    of chords, a trigonometric table that he applied to astronomy.

    If the quadrilateral is given with its four vertices

    order, then the theorem states that:

    where the vertical lines denote the lengths of the line segments between the named

    This relation may be verbally expressed as follows:

    If a quadrilateral is inscribable in a circle then the product of the measures of its diagonals is equal to the

    sum of the products of the measures of the pairs of opposite sides.

    Moreover, the converse of Ptolemy's theorem is also true:

    In a quadrilateral, if the sum of the products of its two pairs of opposite sides is equal to the product of its

    diagonals, then the quadrilateral can be inscribed in a circle.

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    constructed on a grid of equal-distanced points (i.e., points

    coordinates) such that all the polygon's vertices are grid points, Pick's theorem

    area A of this polygon in terms of the number i

    located in the polygon and the number b of lattice points on the

    placed on the polygon's perimeter

    = 7 interior points and b = 8 boundary

    1 = 7 + 4 1 = 10 (square units)

    Note that the theorem as stated above is only valid for simple polygons,

    i.e., ones that consist of a single piece and do not contain "holes". For a polygon that has

    simple closed curves, the slightly more complicated formula

    Ptolemy's theorem

    Ptolemy's theorem is a relation between the four sides and two diagonals of

    (a quadrilateral whose vertices lie on a common circle).

    mathematician Ptolemy (Claudius

    Ptolemaeus) Ptolemy used the theorem as an aid to creating his table

    tric table that he applied to astronomy.

    If the quadrilateral is given with its four vertices A, B, C, and D in

    where the vertical lines denote the lengths of the line segments between the named

    relation may be verbally expressed as follows:

    If a quadrilateral is inscribable in a circle then the product of the measures of its diagonals is equal to the

    sum of the products of the measures of the pairs of opposite sides.

    olemy's theorem is also true:

    In a quadrilateral, if the sum of the products of its two pairs of opposite sides is equal to the product of its

    diagonals, then the quadrilateral can be inscribed in a circle.

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    distanced points (i.e., points

    Pick's theorem provides a

    i of lattice points in the

    i.e., ones that consist of a single piece and do not contain "holes". For a polygon that has h holes, with a

    simple closed curves, the slightly more complicated formula i + b/2 + h 1

    is a relation between the four sides and two diagonals of

    where the vertical lines denote the lengths of the line segments between the named vertices.

    If a quadrilateral is inscribable in a circle then the product of the measures of its diagonals is equal to the

    In a quadrilateral, if the sum of the products of its two pairs of opposite sides is equal to the product of its

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    Varignon's theorem

    Varignon's theorem is a statement in

    geometry by Pierre Varignon that was first

    published in 1731. It deals with the construction of a

    particular parallelogram (Varignon parallelogram

    from an arbitrary quadrangle.

    The midpoints of the sides of an arbitrary

    quadrangle form a parallelogram. If the quadrangle

    is convex or reentrant, i.e. not a crossing

    quadrangle, then the area of the parallelogram is

    half as big as the area of the quadrangle

    Viviani's theorem

    Viviani's theorem, named after Vincenzo Viviani,

    distances from any interior point to the sides of anequilateral triangle

    length of the triangle's altitude.

    Proof

    This proof depends on the readily-proved proposition that the area of

    a triangle is half its base times its height

    side.

    Let ABC be an equilateral triangle whose height is

    Let P be any point inside the triangle, and

    each of A, B, and C, forming three triangles PAB, PBC, and PCA.

    Now, the areas of these triangles are

    sum of these areas is equal to the area of the enclosing triangle. So we can w

    and thus u + s + t = h.

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    Varignon's theorem

    statement in Euclidean

    that was first

    published in 1731. It deals with the construction of a

    Varignon parallelogram)

    The midpoints of the sides of an arbitrary

    form a parallelogram. If the quadrangle

    is convex or reentrant, i.e. not a crossing

    quadrangle, then the area of the parallelogram is

    half as big as the area of the quadrangle.

    Vincenzo Viviani, states that the sum of the

    interior point to the sides of anequilateral triangle equals the

    proved proposition that the area of

    es its heightthat is, half the product of one side with the altitude from that

    Let ABC be an equilateral triangle whose height is h and whose side is a.

    Let P be any point inside the triangle, and u, s, t the distances of P from the sides. Draw a l

    each of A, B, and C, forming three triangles PAB, PBC, and PCA.

    Now, the areas of these triangles are , , and . They exactly fill the enclosing triangle, so the

    sum of these areas is equal to the area of the enclosing triangle. So we can write:

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    that is, half the product of one side with the altitude from that

    the distances of P from the sides. Draw a line from P to

    . They exactly fill the enclosing triangle, so the

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    Barrow's inequalityBarrow's inequality is an inequality

    arbitrary point within a triangle, the vertices of the triangle, and

    certain points on the sides of the triangle.

    Let P be an arbitrary point inside the

    define U, V, and W as the points where the

    and APB intersect the sides BC,CA, AB

    inequality states that

    with equality holding only in the case of an

    Euler's theorem in geometry

    Euler's theorem states that the distance

    expressed as

    where R and r denote the circumradius and inradius respectively (the radii of the above two circles).

    From the theorem follows the Euler inequality

    Weitzenbck's inequality

    Weitzenbck's inequality, named after

    , , , and area , the following inequality holds:

    Equality occurs if and only if the triangle is equilateral.

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    Barrow's inequality inequality relating the distances between

    triangle, the vertices of the triangle, and

    certain points on the sides of the triangle.

    arbitrary point inside the triangle ABC. From P and ABC,

    as the points where the angle bisectors of BPC, CPA,

    AB, respectively. Then Barrow's

    case of an equilateral triangle

    Euler's theorem in geometry

    states that the distance d between the circumcentre and incentre

    denote the circumradius and inradius respectively (the radii of the above two circles).

    Euler inequality:

    Weitzenbck's inequality

    , named after Roland Weitzenbck, states that for a triangle of

    , the following inequality holds:

    Equality occurs if and only if the triangle is equilateral.

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    an

    incentre of a triangle can be

    denote the circumradius and inradius respectively (the radii of the above two circles).

    Roland Weitzenbck, states that for a triangle of side lengths

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