# Week 3: Complex Numbers – Answers

Week 3: Complex Numbers – Day 5 Week 3: Complex Numbers – Day 4

1. The non-real number i raised to what power is one? (Of course, when raised to the zeroth power it is one, but there are other powers as well.)

i2 = -1
i3 = -1 x i = -i
i4 = -i x i = -(-1) = 1

The fourth power of i is one. i is a fourth root of one.

2. Compute the square of:2-1/2(1 + i)

The square is: (2-1/2)2(1 + i)2

= (1/2) x (1 + i) x (1 + i) = 1/2 x (1 + 2i – 1) = 1/2 x 2i = i

The square is i.

3. Compute the cube of: 2-1/2(1 + i)

The cube is:  i x 2-1/2(1 + i)
which is:  2-1/2(i – 1)
which is:  2-1/2(-1 + i)

4. What power of 2-1/2(1 + i) is one? In other words, 2-1/2(1 + i) raised to what power is one?

Because the square is i, and i4 = 1, then:

((2-1/2(1 + i))2)4 = 1

2-1/2(1 + i) raised to the 8th power is 1.

5. Compute the square of: 2-1/2(1 – i)

The square is 1/2 x (1 – i) x (1 – i) = 1/2 x (1 – 2i +(-1)) = 1/2 x (-2i)
= -i.

Week 3: Complex Numbers – Day 3

1. 1 / i

complex conjugate of i is -i.

= 1*(-i) / i*(-i) = -i /1 = -i

(1 divided by i).

2. (2 + 3i)/i

=(2 + 3i)*(-i) / i*(-i) = 3 – 2i

3. (1 + i) / (1 – i)

= (1 + i)*(1 + i) / (1 – i)*(1 + i) = (1 +2i – 1) / (1 – i + i + 1) = 2i/2 = i

4. i / (1 – i)

= i*(1 + i) / (1 – i)*(1 + i) = (i – 1) / (1 – i + i + 1) = -1/2 + i/2

5. (3 – 2i) / (3 + 2i)

= (3 – 2i)*(3 – 2i) / (3 + 2i)*(3 – 2i) = (9 – 6i – 4) / (3 + 6i – 6i + 4) = (5 – 6i) / 7 = 5/7 – 6i/7

Week 3: Complex Numbers – Day 2

Complex Solutions

1. What are the solutions of x2 + 9 = 0?

x2 = -9
Take square roots of both sides:
x = 3i or -3i

2. What are the solutions of (x + 1)2 + 4 = 0?

(x + 1)2 = -4
Take square roots of both sides:
x + 1 = 2i or -2i
Solutions are -1 + 2i or -1 – 2i

3. What are the solutions of x2 – 2x + 10 = 0?

Completing the square: x2 – 2x + 1 – 1 + 10 = 0

(x – 1)2 + 9 = 0
(x – 1)2 = -9
Take square roots of both sides:
x – 1 = 3i or -3i

Solutions: 1 + 3i or 1 – 3i

4. What are the solutions of x3 + 25x = 0?

Factoring: x(x2 + 25) = 0

Solutions are x = 0 and x2 + 25 = 0.
Solving the second equation:

x2 = -25
Take square roots of both sides:
x = 5i or -5i
Three solutions are: 0, 5i, and -5i

5. What are the solutions of (x – 1)3 + 36(x – 1) = 0?

Factoring: (x – 1)((x-1)2 + 36) = 0

Solutions: x – 1 = 0 (x = 1), and solutions of (x-1)2 + 36 = 0

Solving the second equation:
(x – 1)2 = -36
Take square roots of both sides:
(x – 1) = 6i or -6i

x = 1 + 6i or 1 – 6i

Week 3: Complex Numbers – Day 1

Simplify these expressions. The final result should be a number of the form A + Bi.

1. (-1 + i) + (3 + 2i)

= -1 + 3 + i + 2i = 2 + 3i

2. (7 + 2i) + 10

= 17 + 2i

3. (2 + 2i) x 3i

= 6i + 6x(-1) = -6 + 6i

4. (5 + 3i) x (5 – 3i)

= 25 + 15i – 15i + 9 = 34 = 34 + 0i

5. (4 + i) x (1 + 6i)

= 4 + 24i + i – 6 = -2 + 25i