# Modern Geometry Homework.

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1 Modern Geometry Homework. 1. Rigid motions of the line. Let R be the real numbers. We define the distance between x, y R by where is the usual absolute value. distance between x and y = x y z = { z, z 0; z, z < 0. Proposition 1. If a, b R are distinct points and x is an equal distance from a and b, that is x a = x b then x is the midpoint between a and b. That is x = a + b 2. Problem 1. Prove this. Hint: One way is to square both sides of the equation x a = x b to get rid of the absolute values, that is and you can now solve this for x. Corollary 2. If then x = 0. (x a) 2 = (x b) 2 x 1 = x + 1 Proof. This is the special case of Proposition 1 with a = 1 and b = 1. Definition 3. A rigid motion of R is a function T : R R that preserves distances. That is T (x) T (y) = x y for all x, y R. We fist give some elementary properties of rigid motions that follow directly from the definition. Proposition 4. Let S and T be rigid motions. Then the composition S T is also a rigid motion. (That is S(T (x)) S(T (y)) = x y for all x, y R.) Problem 2. Prove this. Proposition 5. If T : R R is a rigid motion that has an inverse T 1, then the inverse T 1 is also a rigid motion. Problem 3. Prove this. We now give some examples.

2 Definition 6. Let a R. Then the translation by a is the map T a (x) = x + a. Proposition 7. Each translation is a rigid motion. Moreover if T = T a is a translation, it has the property it moves each point by the same amount. That is there is a constant c such that Problem 4. Prove this. T (x) x = c Definition 8. Let a R. Then the reflection about a is the map R a (x) = 2a x. Proposition 9. Each reflection is a rigid motion. The reflection R a has the property that R a (a) = a and for any x we have R a (x) a = x a. Problem 5. Prove this and draw some pictures illustrating the property R a (x) a = x a. Proposition 10. The translation T a has T a as in inverse. The reflection R a is its own inverse. Problem 6. Prove this. Proposition 11. The following hold: (a) The composition of two translations is a translation. (b) The composition of a translation with a reflection is a reflection. (c) The composition of a reflection and translation is a reflection. (d) The composition of two reflections is a translation. Problem 7. Prove this by finding formulas for all of the following: (a) T a T b, (b) T a R b, (c) R b T a, (d) R a R b. We now will show that we have found all the rigid motions. Proposition 12. Let S be a rigid motion that has the farther property that Then either S(0) = 0. S(x) = x (for all x, that is S = T 0 is the translation by 0) 2

3 3 or S(x) = x (for all x, that is S = R 0 is the reflection in 0.) Problem 8. Prove this along the following lines. (a) Show that for all x S(x) = ±x where the choice of + or may depend on x. Hint: As S(0) = 0 you can use the the defining property of being a rigid motion to conclude that S(x) = S(x) S(0) = x 0 = x. (b) Either S(1) = 1 or S(x) = 1. (c) If S(1) = 1, then S(x) = x for all x, that is S = T 0. Here is the proof we gave in class, which you do not have to turn in. Assume towards a contradiction, that there is some x 0 such that S(x) x. Then by part (a) S(x) = x. Then can be combined with S(x) S(1) = x 1 S(x) S(1) = x 1 = x + 1 which in turn can be combined with Corollary 2 to conclude that x = 0, contradicting our assumption that x 0. (d) If S(1) = 1, then S(x) = x for all x. That is S = R 0, the reflection in the origin. Hint: This can be done in same way as part (c). (e) Put the pieces together to complete the proof. Theorem 13. If T is a rigid motion of R, then T is either a translation or a reflection. Problem 9. Prove this. Hint: Let S : R R be the map S(x) = T (x) T (0). (a) Show that S is a rigid motion with S(0) = 0. (b) Show that either S(x) = x for all x, that is S = T 0, or S(x) = x for all x, that is S = R 0. (c) Use that T (x) = S(x) + T (0) to finish the proof. And here is a problem to relate this to what we have done before. Problem 10. Show that all translations and reflection are affine maps. 2. Rigid motions of the plane. We first recall some vector algebra. Let a = (a 1, a 2 ) and b = (b 1, b 2 ) be vectors in R 2. Then the inner product of a and b is a b = a 1 b 1 + a 2 b 2.

4 4 and we define the length a vector a to be a = a a = a a2 2. This is the natural definition based on the Pythagorean Theorem as the following figure shows: a 1 (a 1, a 2 ) a 2 Figure 1. The segment from the origin is the hypotenuse of a right triangle with legs of size a 1 and a 2 and therefore the length of the hypotenuse is a a2 2. Problem 11. Draw some pictures of when a is in the second and third quadrant (so that one the other or both of a 1 or a 2 is negative) and explain why the formula is still correct even when some of the components of a are negative. If a and b are vectors the distance is dist( a, b) = (a 1 b 1 ) 2 + (a 2 b 2 ) 2. This can again be motivated by the Pythagorean Theorem. Problem 12. Draw a couple of picture showing why this is the correct definition of the distance between points based on the Pythagorean Theorem. Proposition 14. The distance between points is given by dist( a, b) = a b. Problem 13. Prove this. Here is one way to motivate the definition of the inner product based on:

5 Theorem 15 (The Law of Cosines.). Let a triangle be labeled as in the figure where a, b, and c are side lengths and α, β, and γ are angles. b γ α a Then c a 2 = b 2 + c 2 2bc cos(α) b 2 = a 2 + c 2 2ac cos(β) c 2 = a 2 + b 2 2ab cos(γ). Note in the case that γ = 90, so that the triangle is a triangle, with legs a and b and hypotenuse c, then, as cos(γ) = 0 this becomes c 2 = a 2 + b 2 which is just the Pythagorean theorem. Therefore the Law of Cosines can be thought of as a generalization of the Pythagorean to triangle that need not have a right angle. Problem 14. This the correct version of what messed up in class. Given two vectors a and b consider the the following triangle which has vertices at 0, a and b. Let θ be the angle between a and b. Then side lengths of the triangle are as in the figure. b b a b 0 (a) Show b a 2 = a 2 2 a b + b 2. (b) Explain why θ a b a 2 = a 2 + b 2 2 a b cos θ (c) Use part (a) to show the equation of (b) simplifies to a b = a b cos θ (d) Use this to find the angle between the vector a = 1, 2 and b = 2, 3. (e) If a = b show that the two vector v = a + b and w = a b are orthogonal. Hint: Two vectors are orthogonal if and only if their inner product is zero. β a 5

6 Definition 16. A map T : R 2 R 2 is a rigid motion or isometry iff it preserves distances. That is for all x, y R 2 we have T ( x) T ( y) = x y. Let us give a few basic properties of rigid motions before we give examples. Proposition 17. Let S and T be rigid motions. Then the composition is a rigid motion. If T is a rigid motion and it has an inverse T 1, then T 1 is also a rigid motion. Problem 15. Prove this. Definition 18. Let a R 2. Then the translation by a is the map T a ( x) = x + a. Proposition 19. Each translation T a is an rigid motion. In the case a = 0 this translation is the identity map. Problem 16. Prove this. Definition 20. A 2 2 matrix P is orthogonal iff for all vectors x and y we have (P x) (P y) = x y. This terminology many seem a little strange at first, here part of an explanation. Write p11 p P = 12 p 21 p 22 and let p11 p12 P 1 =, P p 2 = 21 p 22 be the columns of P. Let [ [ 1 0 e 1 =, e 0] 2 = 1] be the standard basis of R 2. Problem 17. We have done this before, but here it is again for review. Show P e 1 = P 1 P e 2 = P 1. That is P e 1 is the first column of P and P e 2 is the second column of P. Proposition 21. If P = [P 1, P 2 ] is a matrix with columns P 1 and P 2 and P is orthogonal, then P 1 and P 2 both have length 1 and P 1 and P 2 are orthogonal to each other. More explicitly P 1 = P 2 = 1, P 1 P 2 = 0. 6

7 Problem 18. Prove this. Hint: By the definition of P being orthogonal we have that for all vectors x and y that (P x) (P y) = x y. We now just need to make smart choices of x and y. For example if x = e 1 and y = e 2, then we have P 1 2 = P 1 P 1 = (P e 1 ) (P e 1 ) = e 1 e 1 = 1. This P 1 = 1 = 1. The rest of the proof works along the same lines. The reason we care about orthogonal matrices is Proposition 22. Let P be an orthogonal matrix. Then for any vector v we have P v = v and therefore for all x and y Therefore P is a rigid motion of R 2. P x P y = x y. Problem 19. Prove this along the following lines. Let P = [P 1, P 2 ] so that P 1 and P 2 are the columns. We have seen P 1 = P 2 = 1 and P 1 P 2 = 0. These are the only properties of P that we will use. Let v1 v = v 1 e 1 + v 2 e 2 =. v 2 (a) Show P v = v 1 P 1 + v 2 P 2. Hint: One what to start is to use basic properties of matrix multiplication: P v = P (v 1 e 1 + v 2 e 2 ) = v 1 P e 1 + v 2 P e 2 and use that P e 1 = P 1 etc. (b) Show P v 2 = v v2 2 = v 2. Hint: One way to start is P v 2 = (P v) (P v) = (v 1 P 1 + v 2 P 2 ) (v 1 P 1 + v 2 P 2 ) = v 2 1P 1 P 1 + 2v 1 v 2 P 1 P 2 + v 2 P 2 P 2 and use what we know about P 1 P 1, P 1 P 2 etc. (c) It should now be easy to see that P v = v. (d) Finally given x and y we have by standard properties of matrices that Thus P x P y = P ( x y). P x P y = P ( x y). Let v = x y and use parts (a), (b), and (c) to complete the proof. 7

8 8 Proposition 23. If P is a matrix such that for all vectors v we have P v = v then P is orthogonal. That is for all x and y we have (P x) (P y) = x y. Problem 20. Prove this along the following lines. (a) Show for any vectors a and b that a b = 1 ( a + 4 b 2 a b 2). (b) Use (a) to show and the basic property that P ( x + y) = P x + P y and P ( x y) = P x P y to show (P x) (P y) = 1 4 ( P ( x + y) 2 P ( x y) 2). (c) Our hypothesis is that P v = v for all v. By first letting v = x + y and then v = x y show P ( x + y) 2 = x + y 2, P ( x y) 2 = x y 2 (d) Complete the proof. Theorem 24. Let P = [P 1, P 2 ] be a matrix with columns P 1 and P 2. Then the following are equivalent. (a) P is orthogonal. (That is (P x) (P y) = x y for all x and y.) (b) The columns of P satisfy P 1 = P 2 = 1 and P 1 P 2 = 0. (c) P v = v for all v. Proof. Assume (a) holds. Then Proposition 21 implies (b) holds. Assume (b) holds. Then we have seen in Problem 19 that (c) holds. Assume (c) holds. Then Proposition 23 shows (a) holds. We therefore have the implications (a) = (b) = (c) = (a) which shows the conditions are equivalent. Theorem 25. Let P be a matrix. Then the map T : R 2 R 2 given by T v := P v is a rigid motion if and only if P is orthogonal. Problem 21. Prove this. Hint: If P is orthogonal then it show not be hard to see that T is a rigid motion. Conversely if T is an isometry we have that P x P y = x y for all x and y. Let y = 0 to get P x = x and now use Theorem 24. We now give examples of some orthogonal maps.

9 Definition 26. For any number α let cos α sin α R α = sin α cos α This is the rotation by α about the origin. Proposition 27. The rotation R α is orthogonal. Problem 22. Prove this. To get a geometric picture of what R α does take a point in the plane and write it in polar form: r cos θ v = r sin θ Then cos α sin α r cos θ R α v = sin α cos α r sin θ r(cos α cos θ + sin α sin θ) = r( sin α cos θ + cos α sin θ) r cos(θ + α) = r sin(θ + α) where we have used the addition formulas for sine and cosine. That is R α maps the point (r cos θ, r sin θ) to the point (r cos(θ + α), r sin(θ + α)). That is the argument of a point (that is the angle it makes with the positive x-axis) is increased by α. Proposition 28. If α and β are numbers R α R β = R α+β. (Note for matrices composition and matrix multiplication are the same thing.) Problem 23. Prove this. Hint: Use the formula for matrix multiplication and the addition formulas for the sine and cosine: cos(α + β) = cos(α) cos(β) sin(α) sin(β) sin(α + β) = cos(α) sin(β) + sin(α) cos(α). Proposition 29. The rotation R 0 is the identity map on R 2. The inverse of R α is R α. Problem 24. Prove this. The matrix R α gives the rotation about the point 0. But it is natural to do a rotation about any point of the plane. Here is how we can reduce the case of a rotation about a general point to a rotation about the origin by use of translations. Problem 25. Let a R 2 and let α be a real number. Let T = T a R α T a. 9

10 (a) Show that T is a rigid motion. Hint: T is a composition of rigid motions so you can use Proposition 17. (b) Show that T ( a) = a. Hint: T a ( a) = 0 and R α ( 0) = 0. (c) More generally show T ( a + v) = a + R α v. (d) Using (c) draw some pictures and explain why T is a rotation of angle α about the point a. 10

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