Complex Numbers. Misha Lavrov. ARML Practice 10/7/2012

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1 Complex Numbers Misha Lavrov ARML Practice 10/7/2012

2 A short theorem Theorem (Complex numbers are weird) 1 = 1. Proof. The obvious identity 1 = 1 can be rewritten as 1 1 = 1 1. Distributing the square root, we get 1 1 =. 1 1 Finally, we can cross-multiply to get 1 1 = 1 1, or 1 = 1.

3 Basic complex number facts Complex numbers are numbers of the form a + b ı, where ı 2 = 1.

4 Basic complex number facts Complex numbers are numbers of the form a + b ı, where ı 2 = 1. We add and multiply complex numbers in the obvious way. Other operations: a + b ı = a b ı (conjugation).

5 Basic complex number facts Complex numbers are numbers of the form a + b ı, where ı 2 = 1. We add and multiply complex numbers in the obvious way. Other operations: a + b ı = a b ı (conjugation). a + b ı = a2 + b 2 (absolute value). Note: z = z z.

6 Basic complex number facts Complex numbers are numbers of the form a + b ı, where ı 2 = 1. We add and multiply complex numbers in the obvious way. Other operations: a + b ı = a b ı (conjugation). a + b ı = a2 + b 2 (absolute value). Note: z = z z. We can identify a complex number a + b ı with the point (a, b) in the plane.

7 Complex number facts, continued Corresponding to polar notation for points (r, θ), complex numbers can be expressed as z = r(cos θ + ı sin θ) = r exp( ıθ).

8 Complex number facts, continued Corresponding to polar notation for points (r, θ), complex numbers can be expressed as z = r(cos θ + ı sin θ) = r exp( ıθ). Multiplication is more natural in this form: r 1 exp( ıθ 1 ) r 2 exp( ıθ 2 ) = (r 1 r 2 ) exp( ı(θ 1 + θ 2 )).

9 Complex number facts, continued Corresponding to polar notation for points (r, θ), complex numbers can be expressed as z = r(cos θ + ı sin θ) = r exp( ıθ). Multiplication is more natural in this form: r 1 exp( ıθ 1 ) r 2 exp( ıθ 2 ) = (r 1 r 2 ) exp( ı(θ 1 + θ 2 )). This has a geometric interpretation: rotation by θ, and scaling by r = z.

10 Complex number geometry Problem (AIME 2000/9.) A function f is defined on the complex numbers by f (z) = (a + b ı)z, where a and b are positive numbers. This function has the property that the image of each point in the complex plane is equidistant from that point and the origin. Given that a + b ı = 8 and that b 2 = m/n, where m and n are positive integers, find m/n. Problem (AIME 1992/10.) Consider the region A in the complex plane that consists of all points z such that both z/40 and 40/z have real and imaginary parts between 0 and 1, inclusive. What is the integer that is nearest the area of A?

11 Solution: AIME 2000/9 If f (z) is equidistant from 0 and z for all z, in particular, f (1) = a + b ı is equidistant from 0 and 1.

12 Solution: AIME 2000/9 If f (z) is equidistant from 0 and z for all z, in particular, f (1) = a + b ı is equidistant from 0 and 1. This is true if and only if a = 1 2. We now need to use a + b ı = 8: 8 = a + b ı = a 2 + b 2 = b

13 Solution: AIME 2000/9 If f (z) is equidistant from 0 and z for all z, in particular, f (1) = a + b ı is equidistant from 0 and 1. This is true if and only if a = 1 2. We now need to use a + b ı = 8: 8 = a + b ı = a 2 + b 2 = b b 2 = =

14 Solution: AIME 2000/9 If f (z) is equidistant from 0 and z for all z, in particular, f (1) = a + b ı is equidistant from 0 and 1. This is true if and only if a = 1 2. We now need to use a + b ı = 8: 8 = a + b ı = a 2 + b 2 = b b 2 = = Why is f (z) equidistant from 0 and z for all z, not just z = 1?

15 Solution: AIME 1992/10 Write z = x + y ı. Then z/40 has real part x/40 and imaginary part y/40. If these are between 0 and 1, then 0 x 40 and 0 y 40.

16 Solution: AIME 1992/10 Write z = x + y ı. Then z/40 has real part x/40 and imaginary part y/40. If these are between 0 and 1, then 0 x 40 and 0 y 40. To deal with 40/z, we write it as 40z/(zz) = 40z/ z 2. So 0 40x 40y x 2 1 and 0 + y 2 x 2 + y 2 1.

17 Solution: AIME 1992/10 Write z = x + y ı. Then z/40 has real part x/40 and imaginary part y/40. If these are between 0 and 1, then 0 x 40 and 0 y 40. To deal with 40/z, we write it as 40z/(zz) = 40z/ z 2. So 0 40x 40y x 2 1 and 0 + y 2 x 2 + y 2 1. We can rewrite the first as x 2 + y 2 40x, or (x 20) 2 + y , or z Similarly, the second becomes z 20 ı 20. The rest is algebra.

18 Applications Problem (Basic fact) Show that given any quadrilateral, the midpoints of its sides form a parallelogram. Problem (Law of cosines) Let a, b, and c be the sides of ABC opposite the vertices A, B, and C respectively. Prove that c 2 = a 2 + b 2 2ab cos C.

19 Basic fact: solution Let a, b, c, and d be the complex numbers corresponding to four vertices of a quadrilateral.

20 Basic fact: solution Let a, b, c, and d be the complex numbers corresponding to four vertices of a quadrilateral. Then the midpoints of the sides are given by a+b a+d 2. 2, b+c 2, c+d 2, and

21 Basic fact: solution Let a, b, c, and d be the complex numbers corresponding to four vertices of a quadrilateral. Then the midpoints of the sides are given by a+b a+d 2. 2, b+c 2, c+d 2, and It s easiest to show that both pairs of opposite sides are congruent. We have: a + b 2 b + c 2 b + c 2 c + d 2 = a c 2 = b d 2 = = c + d 2 a + b 2 a + d 2 a + d 2..

22 Law of cosines: solution We can assume that the three vertices of ABC correspond to complex numbers 0, 1, and z, with the vertex C at 0.

23 Law of cosines: solution We can assume that the three vertices of ABC correspond to complex numbers 0, 1, and z, with the vertex C at 0. Then a = 1, b = z, and c = z 1.

24 Law of cosines: solution We can assume that the three vertices of ABC correspond to complex numbers 0, 1, and z, with the vertex C at 0. Then a = 1, b = z, and c = z 1. Write z = x + y ı = r(cos θ + ı sin θ). Then θ = C, and cos θ = x/ z. Then we have a 2 + b 2 2ab cos θ = 1 + z 2 2 z x z = 1 + z 2 2x.

25 Law of cosines: solution We can assume that the three vertices of ABC correspond to complex numbers 0, 1, and z, with the vertex C at 0. Then a = 1, b = z, and c = z 1. Write z = x + y ı = r(cos θ + ı sin θ). Then θ = C, and cos θ = x/ z. Then we have a 2 + b 2 2ab cos θ = 1 + z 2 2 z x z = 1 + z 2 2x. On the other hand, z 1 2 = (x 1) 2 + y 2 = x 2 2x y 2 = z 2 2x + 1.

26 Roots of unity and polynomials Fact: the equation z n = 1 has n complex roots, which are evenly spaced around the circle z = 1 and start from z = 1. They can be written, for some angle θ = 2πk n, k an integer 0 k < n, as z = cos θ + ı sin θ = exp( ıθ) We can also think of these as follows. Let ω = cos 2π n + ı sin 2π n. Then the roots of z n = 1 are 1, ω, ω 2,..., ω n 1. Problem (AMC 12A 2002/24.) Find the number of ordered pairs of real numbers (a, b) such that (a + b ı) 2002 = a b ı.

27 Solution: AMC 12A 2002/24 Multiplying by a + b ı again, we get (a + b ı) 2003 = a 2 + b 2, or z 2003 = z 2.

28 Solution: AMC 12A 2002/24 Multiplying by a + b ı again, we get (a + b ı) 2003 = a 2 + b 2, or z 2003 = z 2. In particular, z 2003 = z 2, so z can be 0 or 1.

29 Solution: AMC 12A 2002/24 Multiplying by a + b ı again, we get (a + b ı) 2003 = a 2 + b 2, or z 2003 = z 2. In particular, z 2003 = z 2, so z can be 0 or 1. If z = 0, then z = 0 is one solution. If z = 1, then z 2003 = 1, which has 2003 solutions.

30 Roots of unity, continued Problem (HMMT 2010 Algebra/4.) Suppose that there exist nonzero complex numbers a, b, c, d such that z satisfies az 3 + bz 2 + cz + d = 0 and bz 3 + cz 2 + dz + a = 0. Find all possible (complex) values of z. Problem (ARML 1995/T5.) Determine all integer values of θ with 0 θ 90 for which (cos θ + ı sin θ ) 75 is a real number.

31 Solution: HMMT 2010 Algebra/4 Multiplying the first equation by z gives us az 4 + bz 3 + cz 2 + dz = 0. Now we subtract the second equation to get az 4 a = 0. Since a 0, we must have z 4 = 1.

32 Solution: HMMT 2010 Algebra/4 Multiplying the first equation by z gives us az 4 + bz 3 + cz 2 + dz = 0. Now we subtract the second equation to get az 4 a = 0. Since a 0, we must have z 4 = 1. The fourth roots of unity are 1, ı, 1, and ı. We can get all of these except 1 by setting a = b = c = d = 1, so that both equations become z 3 + z 2 + z + 1 = 0.

33 Solution: HMMT 2010 Algebra/4 Multiplying the first equation by z gives us az 4 + bz 3 + cz 2 + dz = 0. Now we subtract the second equation to get az 4 a = 0. Since a 0, we must have z 4 = 1. The fourth roots of unity are 1, ı, 1, and ı. We can get all of these except 1 by setting a = b = c = d = 1, so that both equations become z 3 + z 2 + z + 1 = 0. If z = 1, then we must have a + b + c + d = 1 = 0, but fortunately it s not too hard to find examples of such a, b, c, and d. So all four of the values we found are possible values of z.

34 Solution: ARML 1995/T5 We know that cos θ + ı sin θ is on the circle z = 1, and taking powers of it just rotates it around. The only real numbers it could possibly hit are 1 and 1.

35 Solution: ARML 1995/T5 We know that cos θ + ı sin θ is on the circle z = 1, and taking powers of it just rotates it around. The only real numbers it could possibly hit are 1 and 1. So we could try to solve (cos θ + ı sin θ) 75 = 1 and (cos θ + ı sin θ) 75 = 1 separately. But we can also combine these two into (cos θ + ı sin θ) 150 = 1.

36 Solution: ARML 1995/T5 We know that cos θ + ı sin θ is on the circle z = 1, and taking powers of it just rotates it around. The only real numbers it could possibly hit are 1 and 1. So we could try to solve (cos θ + ı sin θ) 75 = 1 and (cos θ + ı sin θ) 75 = 1 separately. But we can also combine these two into (cos θ + ı sin θ) 150 = 1. There are 150 roots, but we want ones for which 0 θ 90. There are 150/4 = 38 of these. We could write down what they are, but that s boring.

37 Even more roots of unity Problem (AIME 1997/14, modified.) Let v and w be distinct, randomly chosen roots of the equation z = 0. Find the probability that v + w 1. Problem (AIME 1996/11, modified.) Let P be the product of the roots of z 4 + z 3 + z 2 + z + 1 = 0 that have a positive imaginary part, and suppose that P = r(cos θ + ı sin θ ), where r > 0 and 0 θ < 360. Find θ.

38 Solution: AIME 1997/14 We know v and w are points on the circle of radius 1 around 0. The closer together v and w are to each other, the bigger v + w is.

39 Solution: AIME 1997/14 We know v and w are points on the circle of radius 1 around 0. The closer together v and w are to each other, the bigger v + w is. By drawing some triangles, we see that if v and w are 120 or 2 3 π radians apart, then v + w is exactly 1.

40 Solution: AIME 1997/14 We know v and w are points on the circle of radius 1 around 0. The closer together v and w are to each other, the bigger v + w is. By drawing some triangles, we see that if v and w are 120 or 2 3 π radians apart, then v + w is exactly 1. After some counting, we conclude that the probability is

41 Solution: AIME 1997/14 We know v and w are points on the circle of radius 1 around 0. The closer together v and w are to each other, the bigger v + w is. By drawing some triangles, we see that if v and w are 120 or 2 3 π radians apart, then v + w is exactly 1. After some counting, we conclude that the probability is Exercise: in the original AIME problem, you were asked to find the probability that v + w What is the answer then?

42 Solution: AIME 1996/11 We recognize z 4 + z 3 + z 2 + z + 1 as z5 1 z 1. So z 4 + z 3 + z 2 + z + 1 = 0 if z 5 = 1 and yet z 1.

43 Solution: AIME 1996/11 We recognize z 4 + z 3 + z 2 + z + 1 as z5 1 z 1. So z 4 + z 3 + z 2 + z + 1 = 0 if z 5 = 1 and yet z 1. There are four roots: ω = cos 2π 5 + ı sin 2π 5, ω2, ω 3, and ω 4.

44 Solution: AIME 1996/11 We recognize z 4 + z 3 + z 2 + z + 1 as z5 1 z 1. So z 4 + z 3 + z 2 + z + 1 = 0 if z 5 = 1 and yet z 1. There are four roots: ω = cos 2π 5 + ı sin 2π 5, ω2, ω 3, and ω 4. The first two have positive imaginary part, and their product is ω 3, which is cos 6π 5 + ı sin 6π 5. So θ = 6π 5 = 216.

45 Solution: AIME 1996/11 We recognize z 4 + z 3 + z 2 + z + 1 as z5 1 z 1. So z 4 + z 3 + z 2 + z + 1 = 0 if z 5 = 1 and yet z 1. There are four roots: ω = cos 2π 5 + ı sin 2π 5, ω2, ω 3, and ω 4. The first two have positive imaginary part, and their product is ω 3, which is cos 6π 5 + ı sin 6π 5. So θ = 6π 5 = 216. Exercise: in the original problem, we instead had z 6 + z 4 + z 3 + z = 0. How does this change the answer?

46 Hard problems Problem (AIME 1994/13.) The equation z 10 + (13z 1) 10 = 0 has ten complex real roots r 1, r 1,..., r 5, r 5. Find the value of r 1 r 1 r 2 r 2 r 3 r 3 r 4 r 4 r 5 r 5 Problem (AIME 1998/13.) If a 1 < a 2 < < a n is a sequence of real numbers, we define its complex power sum to be a 1 ı + a 2 ı a n ı n. Let S n be the sum of all complex power sums of all nonempty subsequences of 1, 2,..., n.given that S 8 = ı, find S 9.

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