Section 6.1

Angles of Polygons

A diagonal of a polygon is a segment that connects any two nonconsecutive vertices.

The sum of the angle measures of a polygon is the sum of the angle measures of the triangles formed by drawing all the possible diagonals from one vertex.

The vertices of polygon PQRST that are not consecutive with vertex P are vertices R and S. Therefore, polygon PQRST has two diagonals from vertex P, PR and PS. Notice that the diagonals from vertex P separate the polygon into three triangles.

Since the sum of the angle measures of a triangle is 180, we can make a table and look for a pattern to find the sum of the angle measures for any convex polygon.

This leads to the following theorem:

You can use the Polygon Interior Angles Sum Theorem to find the sum of the interior angles of a polygon and to find missing measures in polygons.

Example 1:

a) Find the sum of the measures of the interior angles of a convex nonagon.

A nonagon has nine sides. Use the Polygon Interior Angles Sum Theorem to find the sum of its interior angle measures.

(n – 2) ● 180 = (9 – 2) ● 180 n = 9

= 7 ● 180 or 1260 Simplify.

Answer: The sum of the measures is 1260°.

Example 1: b) Find the measure of each interior angle of parallelogram RSTU.

Since n = 4 the sum of the measures of the interior angles is 180(4 – 2) or 360°. Write an equation to express the sum of the measures of the interior angles of the polygon.

Step 1 Find the value of x.

360 = mÐR + mÐS + mÐT + mÐU Sum of measures of interior angles

360 = 5x + (11x + 4) + 5x + (11x + 4) Substitution

360 = 32x + 8 Combine like terms

352 = 32x Subtract 8 from each side

11 = x Divide each side by 32

Step 2 Use the value of x to find the measure of each angle.

Answer: mR = 55°, mS = 125°, mT = 55°, mU = 125°

mR = 5x= 5(11) or 55°

mS = 11x + 4= 11(11) + 4 or 125°

mT = 5x= 5(11) or 55°

mU = 11x + 4= 11(11) + 4 or 125°

Example 3: ARCHITECTURE A mall is designed so that five walkways meet at a food court that is in the shape of a regular pentagon. Find the measure of one of the interior angles of the pentagon.

Find the sum of the interior angle measures.(n – 2) ● 180 = (5 – 2) ● 180 n = 5 = 3 ● 180 or 540 Simplify.

Find the measure of one interior angle.

Substitution

= 108° Divide.

sum of interior angle measures 540

number of congruent angles 5

Example 3: a) The measure of an interior angle of a regular polygon is 150. Find the number of sides in the polygon.

Given the interior angle measure of a regular polygon, you can also use the Polygon Interior Angles Sum Theorem to find a polygon’s number of sides.

Use the Interior Angle Sum Theorem to write an equation to solve for n, the number of sides.

S = 180(n – 2) Interior Angle Sum Theorem

(150)n = 180(n – 2) S = 150n

150n = 180n – 360 Distributive Property

0 = 30n – 360 Subtract 150n from each side.

360 = 30n Add 360 to each side.

12 = n Divide each side by 30.

Example 3: b) The measure of an interior angle of a regular polygon is 144. Find the number of sides in the polygon.

Use the Interior Angle Sum Theorem to write an equation to solve for n, the number of sides.

S = 180(n – 2) Interior Angle Sum Theorem

(144)n = 180(n – 2) S = 144n

144n = 180n – 360 Distributive Property

0 = 36n – 360 Subtract 144n from each side.

360 = 36n Add 360 to each side.

10 = n Divide each side by 36.

Does a relationship exist between the number of sides a convex polygon and the sum of its exterior angle measures? Examine the polygons below in which an exterior angle has been measured at each vertex

Did you notice that the sum of the exterior angle measures in each case is 360? This suggests the following theorem:

Example 4: a) Find the value of x in the diagram.

5x + (4x – 6) + (5x – 5) + (4x + 3) + (6x – 12) + (2x + 3) + (5x + 5) = 360

(5x + 4x + 5x + 4x + 6x + 2x + 5x) + [(–6) + (–5) + 3 + (–12) + 3 + 5] = 360

31x – 12 = 360

31x = 372

x = 12

Example 4:

b) Find the measure of each exterior angle of a regular decagon.

A regular decagon has 10 congruent sides and 10 congruent angles. The exterior angles are also congruent, since angles supplementary to congruent angles are congruent. Let n = the measure of each exterior angle and write and solve an equation.

10n = 360 Polygon Exterior AngleSum Theorem

n = 36 Divide each side by 10.