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.
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=[(sx−ty)+(sy+tx)i]+[(sx−ty)−(sy+tx)i]=2sx−2ty.
It follows that the line ax+by+d=0 can be represented by the equation
αz+¯αz+d=0
where α=12(a−bi) 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.
Any line in C can be expressed by the equation |z−γ|=|z−β| 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
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Given two points γ and β in C, z is equidistant from both if and only if |z−γ|2=|z−β|2. Expanding this equation, we obtain
(z−γ)(¯z−γ)=(z−β)(¯z−β)|z|2−¯γz−γ¯z+|γ|2=|z|2−¯βz−β¯z+|β|2¯(β−γ)z+(β−γ)¯z+(|γ|2−|β|2)=0.
This last equation has the form of a line, letting α=¯(β−γ) 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 |z−γ|=|z−β| describes the line. We leave the details to the reader.
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 r0≥0, then a complex number z=reiθ satisfies the equation z2=z0 if and only if
reiθ⋅reiθ=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±√β2−4αγ2α.
For instance, z2+2z+4=0 has two solutions:
z=−2±√−122=−1±√3i
since √−1=i.
Consider the equation z2−(3+3i)z=2−3i. To solve this equation for z we first rewrite it as
z2−(3+3i)z−(2−3i)=0.
We use the quadratic formula with α=1, β=−(3+3i), and γ=−(2−3i), to obtain the solution(s)
z=3+3i±√(3+3i)2+4(2−3i)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+6ix2−y2+2xyi=8+6i.
Thus, we have two equations and two unknowns:
x2−y2=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.
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.
If we let z=x+yi and z0=h+ki, then the complex equation
|z−z0|=r
describes the circle in the plane centered at z0 with radius r>0.
To see this, note that
|z−z0|=|(x−h)+(y−k)i|=√(x−h)2+(y−k)2.
So |z−z0|=r is equivalent to the equation (x−h)2+(y−k)2=r2 of the circle centered at z0 with radius r.
For instance, |z−3−2i|=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).
Describe each complex expression below as a region in the plane.
- |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.
- 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.
- 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 y≥0. These points determine a ray on the positive imaginary axis.
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.
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(a−bi) 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
|z−z0|=r.
Exercises
Use a complex variable to describe the equation of the line y=mx+b. Assume m≠0. In particular, show that this line is described by the equation
(m+i)z+(m−i)¯z+2b=0.
In each case, sketch the set of complex numbers z satisfying the given condition.
- |z+i|=3.
- |z+i|=|z−i|.
- Re(z)=1.
- |z/10+1−i|<5.
- Im(z)> Re(z).
- Re(z)=|z−2|.
- Hint
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It may be helpful to set z=x+yi and rewrite the expression in terms of x and y.
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
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Suppose β and γ are complex numbers such that |u−β|=|u−γ|, |v−β|=|v−γ| and |w−β|=|w−γ|. Argue that β and γ must in fact be equal complex numbers.
Find all solutions to the quadratic equation z2+iz−(2+6i)=0.