# impossible numbers

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MAT 320 Spring 2008. Impossible Numbers. Old Problems. You may remember from geometry that you can perform many constructions only using a straightedge and a compass These include drawing circles, constructing right angles, bisecting angles, etc. - PowerPoint PPT Presentation

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• MAT 320 Spring 2008

• You may remember from geometry that you can perform many constructions only using a straightedge and a compass

These include drawing circles, constructing right angles, bisecting angles, etc.

But there are other problems that the ancient Greeks wanted to try to solve with this method

• The Greeks wanted to know if any of the following were possibleTrisecting the angle: Given an angle, divide it into three congruent anglesDoubling the cube: Given a cube, construct another cube with exactly twice the volumeSquaring the circle: Given a circle, create a square with the same area

• It turns out that all of these constructions are impossible

In order to understand why, we need to think about how constructions really work

We say that we can construct a point (x, y) if we can find that point as an intersection of lines or circles that we can construct

• The things we can construct areLines: We can use our straightedge to construct a line between any two pointsCircles: Given two points, we can construct a circle with the center at one point and which passes through the otherPerpendiculars: Given a line and a point, we can construct a perpendicular line that passes through the point

• We say that a number is constructible if it is the x or y-coordinate of a constructible point

For example, all of the integers are constructible

• The number is also constructible, since the point is the intersection of the first two circles on the previous slide

In fact, the set of constructible numbers is closed under addition, subtraction, multiplication, division, and square roots

• The set of constructible numbers forms a field that contains the rational numbers

This field contains only those numbers that can be obtained from (possibly repeatedly) extending Q with the roots of quadratic polynomials

• For example, Gauss showed that

Since this number is constructed out of rational numbers and square roots, this number must be constructible

We can use this fact to construct a regular 17-sided polygon

• Lets think about trisection of an angle, specifically a 60-degree angle

60-degree angles are constructible: cos(60) and sin(60) are both constructible numbers

Using trig identities, its possible to show that cos(20) is a root of the polynomial x3 3x 1

• Since the polynomial for which cos(20) is a root has degree 3, that means that cos(20) will involve cube roots, which arent allowed

So cos(20) is not a constructible number, and 60-degree angles are just one example of angles we cannot trisect with straightedge and compass

• Given a 1 x 1 x 1 cube, we would need to construct a x x cube to have exactly double the volume

But is not a number we can construct, so we wouldnt be able to create a segment exactly units long to create our cube

• Given a circle of radius 1 (and area ), we would need to construct a square whose sides have length the square root of

Even though square roots are allowed, is not a rational number

It turns out is a transcendental number, which means its not the root of any polynomial with rational coefficients

• Another famous impossibility that is related to these ideas is credited to Niels Abel (1802-1829)

He proved that there is no way to solve a generic fifth-degree polynomial using radicals (even allowing 5th roots!)

• Of course, some quintics are solvable using radicals

An example is , whose roots are 1 (twice), -1, i, and i

But what Abel proved is that there is no analogue to the quadratic formula for quintics

• Abels proof is beyond what we have learned in this course, but here are some related ideas

Have you ever noticed that roots of polynomials tend to come in groups?

For example, if you know that is the root of a quadratic, you can be sure that is also a root

• It turns out that this is no accident

The roots of higher degree polynomials are related in more complicated ways, but they are still related

Once the degree reaches 5, the relationships become so complicated that there is sometimes no way to unentangle the roots from one another

• Keep in mind that we can still solve quintic equations using numerical methods

The issue is that some quintic equations have roots that we cannot express with our normal radical notation

One example is x5 x + 1

This does not mean that the roots dont exist as complex numbers