Complex Numbers - Study Guide

Complex Numbers 1 1.1 Introduction Real or Complex? Definition: The complex number z is defined as: z = a + bi where a, b are real numbers and i = Examples: √ (1) −1. (Side note: Engineers typically use j instead of i). 5 + 2i, 3− √ 2i, 3, −5i Real numbers are also complex (by taking b = 0). 1.2 Visualizing Complex Numbers A complex number is defined by it’s two real numbers. If we have z = a + bi, then: Definition: The real part of a + bi is a, Re(z) = Re(a + bi) = a The imaginary part of a + bi is b, Im(z) = Im(a + bi) = b To visualize a complex number, we can plot it on the plane. The horizontal axis is for the real part, and the vertical axis is for the imaginary part; a + bi is plotted as the point (a, b). In Figure 1, we can see that it is also possible to represent the point a + bi, or (a, b) in polar form, by computing its modulus (or size), and angle (or argument): p |z| = a2 + b2 φ = arg(z) We have to be a bit careful defining φ- Being an angle, it is not uniquely described (0 = 2π = 4π, etc). It is customary to restrict φ to be in the interval (−π, π]. Figure 1: Graphically representing the complex number z = x + iy, and visualizing its complex conjugate, z̄ . 1 1.3 1.3.1 Operations on Complex Numbers The Conjugate of a Complex Number If z = a + bi is a complex number, then its conjugate, denoted by z̄ is a − bi. For example, z = 3 + 5i ⇒ z̄ = 3 − 5i z = i ⇒ z̄ = −i z = 3 ⇒ z̄ = 3 Graphically, the conjugate of a complex number is it’s mirror image across the horizontal axis. 1.3.2 Addition/Subtraction, Multiplication/Division To add (or subtract) two complex numbers, add (or subtract) the real parts and the imaginary parts separately: (a + bi) ± (c + di) = (a + c) ± (b + d)i To multiply, expand it as if you were multiplying polynomials: (a + bi)(c + di) = ac + adi + bci + bdi2 = (ac − bd) + (ad + bc)i and simplify using i2 = −1. Note what happens when you multiply a number by its conjugate: z z̄ = (a + bi)(a − bi) = a2 − abi + abi − b2 i2 = a2 + b2 = |z|2 Division by complex numbers z, w: denominator): z w, is defined by translating it to real number division (rationalize the z z w̄ z w̄ = = w ww̄ |w|2 Example: 1 + 2i (1 + 2i)(3 + 5i) −7 11 = = + i 3 − 5i 34 34 34 1.4 1.4.1 The Polar Form of Complex Numbers Euler’s Formula Any point on the unit circle can be written as (cos(θ), sin(θ)), which corresponds to the complex number cos(θ) + i sin(θ). It is possible to show the following directly, but we’ll use it as a definition: Definition (Euler’s Formula): eiθ = cos(θ) + i sin(θ). 1.4.2 Polar Form of a + bi: The polar form is defined as: z = reiθ where r = |z| = p a2 + b2 θ = arg(z) To be sure that the polar form is unique, we restrict θ to be in the interval (−π, π]. You might think of arg(z) as the four-quadrant inverse tangent- That is:
• If (a, b) is in the first or fourth quadrant, then θ = tan−1 ab . • If a = 0 and b 6= 0, then θ is either π/2 (for b > 0) or −π/2.
• If (a, b) is in the second quadrant, add π: θ = tan−1 ab + π
• If (a, b) is in the third quadrant, subtract π: θ = tan−1 ab − π • The argument of zero is not defined. Best way to remember these: Quickly plot a + bi to see if you need to add or subtract π. 2 1.5 Exponentials and Logs The logarithm of a complex number is easy to compute if the number is in polar form:

ln(a + bi) = ln reiθ = ln(r) + ln eiθ = ln(r) + iθ The logarithm of zero is left undefined (as in the real case). However, we can now compute the log of a negative number:
π π or the log of i : ln(i) = ln(1) + i = i ln(−1) = ln 1 · eiπ = iπ 2 2 Note that the usual rules of exponentiation and logarithms still apply. To exponentiate a number, we convert it to multiplication (a trick we used in Calculus when dealing with things like xx ): ab = eb ln(a) Example, 2i = ei ln(2) = cos(ln(2)) + i sin(ln(2)) √ i π/4
1/2 √ Example: 1 + i = (1 + i)1/2 = 2e = (21/4 )ei π/8 i i ln(i) i(iπ/2) −π/2 Example: i = e =e =e 2 Real Polynomials and Complex Numbers If ax2 + bx + c = 0, then the solutions come from the quadratic formula: √ −b ± b2 − 4ac x= 2a In the past, we only took real roots. Now we can use complex roots. For example, the roots of x2 + 1 = 0 are x = i and x = −i. Check: (x − i)(x + i) = x2 + xi − xi − i2 = x2 + 1 Some facts about polynomials when we allow complex roots: 1. An nth degree polynomial can always be factored into n roots. (Unlike if we only have real roots!) This is the Fundamental Theorem of Algebra. 2. If a + bi is a root to a real polynomial, then a − bi must also be a root. This is sometimes referred to as “roots must come in conjugate pairs”. 3 Exercises 1. Suppose the roots to a cubic polynomial are a = 3, b = 1−2i and c = 1+2i. Compute (x−a)(x−b)(x−c). 2. Find the roots to x2 − 2x + 10. Write them in polar form. 3. Show that: z + z̄ 2 4. For the following, let z1 = −3 + 2i, z2 = −4i Re(z) = Im(z) = z − z̄ 2i (a) Compute z1 z̄2 , z2 /z1 (b) Write z1 and z2 in polar form. 5. In each problem, rewrite each of the following in the form a + bi: (a) (b) (c) (d) (e) (f) e1+2i e2−3i eiπ 21−i π e2− 2 i πi 3