Processing math: 100%
Skip to main content

Section 2.2 Polar Form of a Complex Number

A point (x,y) in the plane can be represented in polar form (r,θ) according to the relationships in Figure 2.2.1.

Figure 2.2.1. Polar coordinates of a point in the plane

Using these relationships, we can rewrite

x+yi=rcos(θ)+rsin(θ)i=r(cos(θ)+isin(θ)).

This leads us to make the following definition. For any real number θ, we define

eiθ=cos(θ)+isin(θ).

For instance, eiπ/2=cos(π/2)+isin(π/2)=0+i⋅1=i.

Similarly, ei0=cos(0)+isin(0)=1, and it's a quick check to see that eiπ=−1, which leads to a simple equation involving the most famous numbers in mathematics (except 8), truly an all-star equation:

eiπ+1=0.

If z=x+yi and (x,y) has polar form (r,θ) then z=reiθ is called the polar form of z. The non-negative scalar |r| is the modulus of z, and the angle θ is called the argument of z, denoted arg(z).

Example 2.2.2. Exploring the polar form.

On the left side of the following diagram, we plot the points z=2eiπ/4,w=3eiπ/2,v=−2eiπ/6,u=3e−iπ/3.

To convert z=−3+4i to polar form, refer to the right side of the diagram. We note that r=√9+16=5, and tan(α)=4/3, so θ=π−tan−1(4/3)≈2.21 radians. Thus,

−3+4i=5ei(π−tan−1(4/3))≈5e2.21i.

We use the definition of the complex exponential and some trigonometric identities.

\begin{align*} re^{i\theta}\cdot se^{i\beta} \amp = r(\cos\theta + i\sin\theta)\cdot s(\cos\beta+i\sin\beta)\\ \amp = (rs)(\cos\theta + i\sin\theta)\cdot (\cos\beta+i\sin\beta)\\ \amp = rs[\cos\theta\cos\beta - \sin\theta\sin\beta + (\cos\theta\sin\beta+\sin\theta\cos\beta)i]\\ \amp = rs[\cos(\theta+\beta) + \sin(\theta+\beta)i]\\ \amp =rs[e^{i(\theta+\beta)}]\text{.} \end{align*}

Thus, the product of two complex numbers is obtained by multiplying their magnitudes and adding their arguments, and

arg(zw)=arg(z)+arg(w),

where the equation is taken modulo 2Ï€. That is, depending on our choices for the arguments, we have arg(vw)=arg(v)+arg(w)+2Ï€k for some integer k.

Example 2.2.4. Polar form with r≥0.

When representing a complex number z in polar form as z=reiθ, we may assume that r is non-negative. If r<0, then

reiθ=−|r|eiθ=(eiÏ€)â‹…|r|eiθ  since âˆ’1=eiÏ€=|r|ei(θ+Ï€),  by Theorem 2.2.3.

Thus, by adding π to the angle if necessary, we may always assume that z=reiθ where r is non-negative.

Exercises Exercises

1.

Convert the following points to polar form and plot them: 3+i, −1−2i, 3−4i, 7,002,001, and −4i.

2.

Express the following points in Cartesian form and plot them: z=2eiπ/3, w=−2eiπ/4, u=4ei5π/3, and z⋅u.

3.

Modify the all-star equation to involve 8. In particular, write an expression involving e,i,Ï€,1, and 8, that equals 0. You may use no other numbers, and certainly not 3.

4.

If z=reiθ, prove that ¯z=re−iθ.