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Product and Quotient Theorems

Multiplying and dividing complex numbers in polar form.

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Product and Quotient Theorems

Complex numbers are found in real world calculations involving: quantum mechanics, signal analysis, fluid dynamics, control theory, and many other fields.

In electrical engineering, complex numbers are used for calculations involving impedance (the resistance to electric flow in a circuit).

Electrical engineers are familiar with the formula:

by comparing it to the similar expression below, which is explored in this lesson, can you identify the variable j?

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Guidance

The Product Theorem

Since complex numbers can be transformed in polar form, the multiplication of complex numbers can also be done in polar form. Suppose we know z1 = r1 (cos θ1 + i sin θ1) z2 = r2 (cos θ2 + i sin θ2)

To multiply the two complex numbers in polar form:

(Use i2 = -1 Gather like terms, factor out i, Substitute the angle sum formulas for both sine and cosine)

This last equation states that the product of two complex numbers in polar form can be obtained by multiplying the polar r values of each of the complex numbers and then multiplying that value by cis of the sum of each of the two angles of the individual complex numbers. This is more concise than the rectangular form for multiplication of complex numbers.

Quotient Theorem

Dividing complex numbers in polar form can be shown using a similar proof that was used to show multiplication of complex numbers. Here we omit the proof and give the result. For z1 = r1(cos θ1 + i sin θ1) and z2 = r2(cos θ2 + i sin θ2), then

Example A

Multiply where and

Solution

For z1,

and

Note that is in the first quadrant since a, and b > 0.

For ,

and

Now we can use the formula

Substituting gives:

So we have

Re-writing in approximate decimal form:

5.656 (0.966 – 0.259i)

5.46 - 1.46i

If the problem was done using only rectangular units then

Gathering like terms and using i2 = -1

or

Example B

Using polar multiplication, find the product

Solution

Let and

and

and

For θ1, first find

Since x > 0 and y < 0 we know that θ1 is in the in the 4th quadrant:

For θ2,

Since θ2 is in the first quadrant,

Using polar multiplication,

subtracting 2π from the augment:

or in expanded form:

In decimal form this becomes: 55.426(0.866 + 0.500i) or 48 + 27.713i

Check:

Example C

Using polar division, find the quotient of

Solution

Given that

for or and , so (4th quadrant)

for or and , so (3rd quadrant)

Using the formula, or

Check by using the complex conjugate to do the division in rectangular form:

The two radically different approaches yield the same answer. The small difference between the two answers is a result of decimal rounding.

Concept question wrap-up

Electrical engineers are familiar with the formula:

by comparing it to the similar expression below, which is explored in this lesson, can you identify the variable j?

In electrical calculations, the letter I is commonly used to denote current, therefore imaginary numbers are identified with a j.

Note the similar usage of i in

Vocabulary

The product theorem is used for the multiplication of complex numbers.

The quotient theorem is used for the division of complex numbers.

Complex conjugates are pairs of complex binomials which result in a single real number product.

Guided Practice

1) Find the product:

2) Find the quotient:

Solutions

1) This one is easier than it looks: Recall

a) ....... By substitution and multplication
b) ..... Substitute
..... Find common denominators
..... Simplify

is the product

2) First finding the quotient by polar multiplication:

since the angle is in the 1st quadrant

for ,

since θ2 is in the 4th quadrant, between 4.712 and 6.282 radians (or 2π)

Finally, using the division formula,

If we assume that , then

Practice

1. Find the product using polar form:
2. Multiply:
3. Divide: 3cis(130o) ÷ 4cis(270o)

If and find:

If and find:

Find the products

1. Find the product using polar form:

Find the quotients

Vocabulary Language: English

Complex Conjugate

Complex Conjugate

Complex conjugates are pairs of complex binomials. The complex conjugate of $a+bi$ is $a-bi$. When complex conjugates are multiplied, the result is a single real number.
complex number

complex number

A complex number is the sum of a real number and an imaginary number, written in the form $a + bi$.
rectangular form

rectangular form

The rectangular form of a point or a curve is given in terms of $x$ and $y$ and is graphed on the Cartesian plane.