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2.4: Complex Expressions

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In this section we look at some equations and inequalities that will come up throughout the text.

Example 2.4.1: Line Equations

The standard form for the equation of a line in the xy-plane is ax+by+d=0. This line may be expressed via the complex variable z=x+yi. For an arbitrary complex number β=s+ti, note that

βz+¯βz=[(sxty)+(sy+tx)i]+[(sxty)(sy+tx)i]=2sx2ty.

It follows that the line ax+by+d=0 can be represented by the equation

αz+¯αz+d=0

where α=12(abi) is a complex constant and d is a real number.

Conversely, for any complex number α and real number d, the equation

αz+¯αz+d=0

determines a line in C.

We may also view any line in C as the collection of points equidistant from two given points.

Theorem 2.4.1

Any line in C can be expressed by the equation |zγ|=|zβ| for suitably chosen points γ and β in C, and the set of all points (Euclidean) equidistant from distinct points γ and β forms a line.

Proof

Given two points γ and β in C, z is equidistant from both if and only if |zγ|2=|zβ|2. Expanding this equation, we obtain

(zγ)(¯zγ)=(zβ)(¯zβ)|z|2¯γzγ¯z+|γ|2=|z|2¯βzβ¯z+|β|2¯(βγ)z+(βγ)¯z+(|γ|2|β|2)=0.

This last equation has the form of a line, letting α=¯(βγ) and d=|γ|2|β|2.

Conversely, starting with a line we can find complex numbers γ and β that do the trick. In particular, if the given line is the perpendicular bisector of the segment γβ, then |zγ|=|zβ| describes the line. We leave the details to the reader.

Example 2.4.2: Quadratic Equations

Suppose z0 is a complex constant and consider the equation z2=z0. A complex number z that satisfies this equation will be called a square root of z0, and will be written as z0.

If we view z0=r0eiθ0 in polar form with r00, then a complex number z=reiθ satisfies the equation z2=z0 if and only if

reiθreiθ=r0eiθ0.

In other words, z satisfies the equation if and only if r2=r0 and 2θ=θ0 (modulo 2π).

As long as r0 is greater than zero, we have two solutions to the equation, so that z0 has two square roots:

±r0eiθ0/2.

For instance, z2=i has two solutions. Since i=1eiπ/2, i=±eiπ/4. In Cartesian form, i=±(22+22i).

More generally, the complex quadratic equation αz2+βz+γ=0 where α, β, and γ are complex constants, will have one or two solutions. This marks an important difference from the real case, where a quadratic equation might not have any real solutions. In both cases we may use the quadratic formula to hunt for roots, and in the complex case we have solutions

z=β±β24αγ2α.

For instance, z2+2z+4=0 has two solutions:

z=2±122=1±3i

since 1=i.

Example 2.4.3: Solving a Quadratic Equation

Consider the equation z2(3+3i)z=23i. To solve this equation for z we first rewrite it as

z2(3+3i)z(23i)=0.

We use the quadratic formula with α=1, β=(3+3i), and γ=(23i), to obtain the solution(s)

z=3+3i±(3+3i)2+4(23i)2z=3+3i±8+6i2.

To determine the solutions in Cartesian form, we need to evaluate 8+6i. We offer two approaches. The first approach considers the following task: Set x+yi=8+6i and solve for x and y directly by squaring both sides to obtain a system of equations.

x+yi=8+6i(x+yi)2=8+6ix2y2+2xyi=8+6i.

Thus, we have two equations and two unknowns:

x2y2=8

2xy=6.

In fact, we also know that x2+y2=|x+yi|2=|(x+yi)2|=|8+6i|=10, giving us a third equation

x2+y2=10.

Adding equations (???) and (???) yields x2=9 so x=±3. Substituting x=3 into equation (???) yields y=1; substituting x=3 into (???) yields y=1. Thus we have two solutions:

8+6i=±(3+i).

We may also use the polar form to determine 8+6i. Consider the right triangle determined by the point 8+6i=10eiθ pictured in the following diagram.

im-square-root.svg

We know 8+6i=±10eiθ/2, so we want to find θ2. Well, we can determine tan(θ2) easily enough using the half-angle formula

tan(θ2)=sin(θ)1+cos(θ).

The right triangle in the diagram shows us that sin(θ)=35 and cos(θ)=45, so tan(θ2)=13. This means that any point reiθ/2 lives on the line through the origin having slope 13, and can be described by k(3+i) for some scalar k. Since 8+6i has this form, it follows that 8+6i=k(3+i) for some k. Since |8+6i|=10, it follows that |k(3+i)|=10, so k=±1. In other words, 8+6i=±(3+i).

Now let's return to the solution of the original quadratic equation in this example:

z=3+3i±8+6i2z=3+3i±(3+i)2.

Thus, z=3+2i or z=i.

Example 2.4.4: Circle Equations

If we let z=x+yi and z0=h+ki, then the complex equation

|zz0|=r

describes the circle in the plane centered at z0 with radius r>0.

To see this, note that

|zz0|=|(xh)+(yk)i|=(xh)2+(yk)2.

So |zz0|=r is equivalent to the equation (xh)2+(yk)2=r2 of the circle centered at z0 with radius r.

For instance, |z32i|=3 describes the set of all points that are 3 units away from 3+2i. All such z form a circle of radius 3 in the plane, centered at the point (3,2).

Example 2.4.5: Complex Expressions as Regions

Describe each complex expression below as a region in the plane.

  1. |1z|>2.

    Taking the reciprocal of both sides, we have |z|<12, which is the interior of the circle centered at 0 with radius 12.

  2. Im(z)< Re(z).

    Set z=x+yi in which case the inequality becomes y<x. This inequality describes all points in the plane under the line y=x, as pictured below.

  3. Im(z)=|z|.

    Setting z=x+yi, this equation is equivalent to y=y2+x2. Squaring both sides we obtain 0=x2, so that x=0. It follows that y=y2=|y| so the equation describes the points (0,y) with y0. These points determine a ray on the positive imaginary axis.

im-plane-region.svg

Moving forward, lines and circles will be especially important objects for us, so we end the section with a summary of their descriptions in the complex plane.

Note: Lines and Circles in C.

Lines and circles in the plane can be expressed with a complex variable z=x+yi.

  • The line ax+by+d=0 in the plane can be represented by the equation

αz+¯αz+d=0

where α=12(abi) is a complex constant and d is a real number.

  • The circle in the plane centered at z0 with radius r>0 can be represented by the equation

|zz0|=r.

Exercises

Exercise 2.4.1

Use a complex variable to describe the equation of the line y=mx+b. Assume m0. In particular, show that this line is described by the equation

(m+i)z+(mi)¯z+2b=0.

Exercise 2.4.2

In each case, sketch the set of complex numbers z satisfying the given condition.

  1. |z+i|=3.
  2. |z+i|=|zi|.
  3. Re(z)=1.
  4. |z/10+1i|<5.
  5. Im(z)> Re(z).
  6. Re(z)=|z2|.
Hint

It may be helpful to set z=x+yi and rewrite the expression in terms of x and y.

Exercise 2.4.3

Suppose u,v,w are three complex numbers not all on the same line. Prove that any point z in C is uniquely determined by its distances from these three points.

Hint

Suppose β and γ are complex numbers such that |uβ|=|uγ|, |vβ|=|vγ| and |wβ|=|wγ|. Argue that β and γ must in fact be equal complex numbers.

Exercise 2.4.4

Find all solutions to the quadratic equation z2+iz(2+6i)=0.


This page titled 2.4: Complex Expressions is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Michael P. Hitchman via source content that was edited to the style and standards of the LibreTexts platform.

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