SAT Math : How to multiply complex numbers

Study concepts, example questions & explanations for SAT Math

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Example Questions

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Example Question #1 : How To Multiply Complex Numbers

Find the product of (3 + 4i)(4 - 3i) given that i is the square root of negative one.

Possible Answers:
7 + i
24
0
24 + 7i
12 - 12i
Correct answer: 24 + 7i
Explanation:

Distribute (3 + 4i)(4 - 3i)

3(4) + 3(-3i) + 4i(4) + 4i(-3i)

12 - 9i + 16i -12i2

12 + 7i - 12(-1)

12 + 7i + 12

24 + 7i

 

Example Question #21 : Complex Numbers

 has 4 roots, including the complex numbers.  Take the product of  with each of these roots.  Take the sum of these 4 results.  Which of the following is equal to this sum?

Possible Answers:

The correct answer is not listed.

Correct answer:

Explanation:

This gives us roots of 

 

The product of  with each of these gives us:

The sum of these 4 is:

 

What we notice is that each of the roots has a negative.  It thus makes sense that they will all cancel out.  Rather than going through all the multiplication, we can instead look at the very beginning setup, which we can simplify using the distributive property:

Example Question #192 : Exponents

Simplify:

Possible Answers:

None of the other responses gives the correct answer.

Correct answer:

Explanation:

Apply the Power of a Product Property:

A power of  can be found by dividing the exponent by 4 and noting the remainder. 6 divided by 4 is equal to 1, with remainder 2, so 

Substituting, 

.

Example Question #1 : How To Multiply Complex Numbers

Multiply  by its complex conjugate.

Possible Answers:

None of the other responses gives the correct answer.

Correct answer:

Explanation:

The complex conjugate of a complex number  is . The product of the two is the number 

.

Therefore, the product of  and its complex conjugate  can be found by setting  and  in this pattern:

,

the correct response.

Example Question #2 : How To Multiply Complex Numbers

Multiply  by its complex conjugate.

Possible Answers:

Correct answer:

Explanation:

The complex conjugate of a complex number  is . The product of the two is the number 

.

Therefore, the product of  and its complex conjugate  can be found by setting  and  in this pattern:

,

the correct response.

Example Question #6 : How To Multiply Complex Numbers

What is the product of  and its complex conjugate?

Possible Answers:

The correct response is not among the other choices.

Correct answer:

The correct response is not among the other choices.

Explanation:

The complex conjugate of a complex number  is , so  has  as its complex conjugate. 

The product of  and  is equal to , so set  in this expression, and evaluate:

.

This is not among the given responses.

Example Question #7 : How To Multiply Complex Numbers

Multiply and simplify:

Possible Answers:

None of the other choices gives the correct response.

Correct answer:

None of the other choices gives the correct response.

Explanation:

The two factors are both square roots of negative numbers, and are therefore imaginary. Write both in terms of  before multiplying:

Therefore, using the Product of Radicals rule:

 

Example Question #24 : Complex Numbers

Evaluate 

Possible Answers:

Correct answer:

Explanation:

 is recognizable as the cube of the binomial . That is,

Therefore, setting  and  and evaluating:

.

Example Question #9 : How To Multiply Complex Numbers

Evaluate 

Possible Answers:

None of the other choices gives the correct response.

Correct answer:

Explanation:

 is recognizable as the cube of the binomial . That is,

Therefore, setting  and  and evaluating:

Applying the Power of a Product Rule and the fact that :

,

the correct value.

Example Question #10 : How To Multiply Complex Numbers

Raise  to the power of 3.

Possible Answers:

Correct answer:

Explanation:

To raise any expression  to the third power, use the pattern

Setting :

Taking advantage of the Power of a Product Rule:

Since ,

and

:

Collecting real and imaginary terms:

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