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# 5.9: Multiplying and Dividing Complex Numbers

Difficulty Level: At Grade Created by: CK-12
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Practice Products and Quotients of Complex Numbers

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Mr. Marchez draws a triangle on the board. He labels the height (2 + 3i) and the base (2 - 4i). "Find the area of the triangle," he says. (Recall that the area of a triangle is A=12bh\begin{align*}A = \frac{1}{2}bh\end{align*}, b is the length of the base and h is the length of the height.)

### Watch This

First watch this video.

Then watch this video.

### Guidance

When multiplying complex numbers, FOIL the two numbers together (see Factoring when a=1\begin{align*}a = 1\end{align*} concept) and then combine like terms. At the end, there will be an i2\begin{align*}i^2\end{align*} term. Recall that i2=1\begin{align*}i^2=-1\end{align*} and continue to simplify.

#### Example A

Simplify:

a) 6i(14i)\begin{align*}6i(1-4i)\end{align*}

b) (52i)(3+8i)\begin{align*}(5-2i)(3+8i)\end{align*}

Solution:

a) Distribute the 6i\begin{align*}6i\end{align*} to both parts inside the parenthesis.

6i(14i)=6i24i2\begin{align*}6i(1-4i)=6i-24i^2\end{align*}

Substitute i2=1\begin{align*}i^2 = -1\end{align*} and simplify further.

=6i24(1)=24+6i\begin{align*}&=6i-24(-1)\\ &=24+6i\end{align*}

Remember to always put the real part first.

b) FOIL the two terms together.

(52i)(3+8i)=15+40i6i16i2=15+34i16i2\begin{align*}(5-2i)(3+8i) &= 15+40i-6i-16i^2\\ &= 15+34i-16i^2\end{align*}

Substitute i2=1\begin{align*}i^2 = -1\end{align*} and simplify further.

=15+34i16(1)=15+34i+16=31+34i\begin{align*}&= 15+34i-16(-1)\\ &= 15+34i+16\\ &= 31+34i\end{align*}

### More Guidance

Dividing complex numbers is a bit more complicated. Similar to irrational numbers, complex numbers cannot be in the denominator of a fraction. To get rid of the complex number in the denominator, we need to multiply by the complex conjugate. If a complex number has the form a+bi\begin{align*}a + bi\end{align*}, then its complex conjugate is abi\begin{align*}a-bi\end{align*}. For example, the complex conjugate of 6+5i\begin{align*}-6 + 5i\end{align*} would be 65i\begin{align*}-6-5i\end{align*}. Therefore, rather than dividing complex numbers, we multiply by the complex conjugate.

#### Example B

Simplify 83i6i\begin{align*}\frac{8-3i}{6i}\end{align*}.

Solution: In the case of dividing by a pure imaginary number, you only need to multiply the top and bottom by that number. Then, use multiplication to simplify.

83i6i6i6i=48i18i236i2=18+48i36=1836+4836i=1243i\begin{align*}\frac{8-3i}{6i}\cdot \frac{6i}{6i} &= \frac{48i-18i^2}{36i^2}\\ &= \frac{18+48i}{-36}\\ &= \frac{18}{-36}+\frac{48}{-36}i\\ &= -\frac{1}{2}-\frac{4}{3}i\end{align*}

When the complex number contains fractions, write the number in standard form, keeping the real and imaginary parts separate. Reduce both fractions separately.

#### Example C

Simplify 35i2+9i\begin{align*}\frac{3-5i}{2+9i}\end{align*}.

Solution: Now we are dividing by 2+9i\begin{align*}2 + 9i\end{align*}, so we will need to multiply the top and bottom by the complex conjugate, 29i\begin{align*}2-9i\end{align*}.

35i2+9i29i29i=627i10i+45i2418i+18i81i2=637i454+81=3937i85=39853785i\begin{align*}\frac{3-5i}{2+9i}\cdot \frac{2-9i}{2-9i} &= \frac{6-27i-10i+45i^2}{4-18i+18i-81i^2}\\ &= \frac{6-37i-45}{4+81}\\ &= \frac{-39-37i}{85}\\ &= - \frac{39}{85}-\frac{37}{85}i\end{align*}

Notice, by multiplying by the complex conjugate, the denominator becomes a real number and you can split the fraction into its real and imaginary parts.

In both Examples B and C, substitute i2=1\begin{align*}i^2 = -1\end{align*} to simplify the fraction further. Your final answer should never have any power of i\begin{align*}i\end{align*} greater than 1.

Intro Problem Revisit The area of the triangle is (2+3i)(24i)2\begin{align*} \frac{(2 + 3i)(2 - 4i)}{2}\end{align*} so FOIL the two terms together and divide by 2.

(2+3i)(24i)=48i+6i12i2=42i12i2\begin{align*}(2 + 3i)(2 - 4i) = 4 - 8i + 6i -12i^2\\ &= 4 - 2i - 12i^2\end{align*}

Substitute i2=1\begin{align*}i^2 = -1\end{align*} and simplify further.

=42i12(1)=42i+12=162i\begin{align*}&= 4 - 2i -12(-1)\\ &= 4 - 2i + 12\\ &= 16 - 2i\end{align*}

Now divide this product by 2.

162i2=8i\begin{align*} \frac {16 - 2i}{2} = 8 - i\end{align*}

Therefore the area of the triangle is 8i\begin{align*}8 -i\end{align*}.

### Guided Practice

1. What is the complex conjugate of 75i\begin{align*}7-5i\end{align*}?

Simplify the following complex expressions.

2. (74i)(6+2i)\begin{align*}(7-4i)(6+2i)\end{align*}

3. 10i5i\begin{align*}\frac{10-i}{5i}\end{align*}

4. 8+i64i\begin{align*}\frac{8+i}{6-4i}\end{align*}

1. 7+5i\begin{align*}7 + 5i\end{align*}

2. FOIL the two expressions.

(74i)(6+2i)=42+14i24i8i2=4210i+8=5010i\begin{align*}(7-4i)(6+2i) &= 42+14i-24i-8i^2\\ &= 42-10i+8\\ &= 50-10i\end{align*}

3. Multiply the numerator and denominator by 5i\begin{align*}5i\end{align*}.

10i5i5i5i=50i5i225i2=5+50i25=525+5025i=152i\begin{align*}\frac{10-i}{5i} \cdot \frac{5i}{5i} &= \frac{50i-5i^2}{25i^2}\\ &= \frac{5+50i}{-25}\\ &= \frac{5}{-25}+\frac{50}{-25}i\\ &= -\frac{1}{5}-2i\end{align*}

4. Multiply the numerator and denominator by the complex conjugate, 6+4i\begin{align*}6 + 4i\end{align*}.

8+i64i6+4i6+4i=48+32i+6i+4i236+24i24i16i2=48+38i436+16=44+38i52=4452+3852i=1113+1926i\begin{align*}\frac{8+i}{6-4i} \cdot \frac{6+4i}{6+4i} &= \frac{48+32i+6i+4i^2}{36+24i-24i-16i^2}\\ &= \frac{48+38i-4}{36+16}\\ &= \frac{44+38i}{52}\\ &= \frac{44}{52} + \frac{38}{52}i\\ &= \frac{11}{13}+\frac{19}{26}i\end{align*}

### Vocabulary

Complex Conjugate

The “opposite” of a complex number. If a complex number has the form a+bi\begin{align*}a+bi\end{align*}, its complex conjugate is abi\begin{align*}a-bi\end{align*}. When multiplied, these two complex numbers will produce a real number.

### Practice

1. i(27i)\begin{align*}i(2-7i)\end{align*}
2. \begin{align*}8i(6+3i)\end{align*}
3. \begin{align*}-2i(11-4i)\end{align*}
4. \begin{align*}(9+i)(8-12i)\end{align*}
5. \begin{align*}(4+5i)(3+16i)\end{align*}
6. \begin{align*}(1-i)(2-4i)\end{align*}
7. \begin{align*}4i(2-3i)(7+3i)\end{align*}
8. \begin{align*}(8-5i)(8+5i)\end{align*}
9. \begin{align*}\frac{4+9i}{3i}\end{align*}
10. \begin{align*}\frac{6-i}{12i}\end{align*}
11. \begin{align*}\frac{7+12i}{-5i}\end{align*}
12. \begin{align*}\frac{4-2i}{6-6i}\end{align*}
13. \begin{align*}\frac{2-i}{2+i}\end{align*}
14. \begin{align*}\frac{10+8i}{2+4i}\end{align*}
15. \begin{align*}\frac{14+9i}{7-20i}\end{align*}

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### Vocabulary Language: English

complex number

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

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