ACT Math : Complex Numbers

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : Complex Numbers

Subtract \(\displaystyle a\) from \(\displaystyle b\), given:

\(\displaystyle a = 3 + i\)

\(\displaystyle b = 4 - 2i\)

Possible Answers:

\(\displaystyle 1+ 3i\)

\(\displaystyle 3 + i\)

\(\displaystyle 1 - 3i\)

\(\displaystyle -1 - 3i\)

\(\displaystyle -1 + 3i\)

Correct answer:

\(\displaystyle 1 - 3i\)

Explanation:

A complex number is a combination of a real and imaginary number. To subtract complex numbers, subtract each element separately.

In equation \(\displaystyle a\)\(\displaystyle 3\) is the real component and \(\displaystyle 1\) is the imaginary component (designated by \(\displaystyle i\)). In equation \(\displaystyle b\)\(\displaystyle 4\) is the real component and \(\displaystyle -2\) is the imaginary component. Solving for \(\displaystyle b - a\),

\(\displaystyle b-a = \left ( 4-2i\right ) - \left ( 3 + i\right ) = \left ( 4-3\right ) + i\left ( -2-1\right ) = 1 - 3i\)

Example Question #2 : Complex Numbers

Simplify the exponent,

\(\displaystyle (3^{6})^{2}\).

Possible Answers:

\(\displaystyle 3^{8}\)

\(\displaystyle 3^{3}\)

\(\displaystyle 3^{4}\)

\(\displaystyle 3^{12}\)

Correct answer:

\(\displaystyle 3^{12}\)

Explanation:

When you have an exponent on the outside of parentheses while another is on the inside of the parentheses, such as in \(\displaystyle (3^{6})^{2}\), multiply the exponents together to get the answer: \(\displaystyle 3^{12}\).

 

This is different than when you have two numbers with the same base multiplied together, such as in \(\displaystyle x^{2} \cdot x^{3} = x^{5}\). In that case, you add the exponents together.

Example Question #3 : Complex Numbers

Complex numbers take the form \(\displaystyle a+bi\), where \(\displaystyle a\) is the real term in the complex number and \(\displaystyle bi\) is the nonreal (imaginary) term in the complex number.

Simplify:

\(\displaystyle (2-2i) - (4-i)\)

Possible Answers:

\(\displaystyle -2-3i\)

\(\displaystyle \textup{None of these}\)

\(\displaystyle 6-3i\)

\(\displaystyle 6-i\)

\(\displaystyle -2-i\)

Correct answer:

\(\displaystyle -2-i\)

Explanation:

Solving this equation is very similar to solving a linear binomial like \(\displaystyle (a+bx)\). To solve, just combine like terms, being careful to watch for double negatives.

 

\(\displaystyle (2-2i) - (4-i)\)

\(\displaystyle (2-4) + (-2i-(-i))\)

\(\displaystyle -2 -(-2i+i)\)

\(\displaystyle -2 -i\)

Example Question #4 : Complex Numbers

Complex numbers take the form \(\displaystyle a+bi\), where \(\displaystyle a\) is the real term in the complex number and \(\displaystyle bi\) is the nonreal (imaginary) term in the complex number.

Which of the following is incorrect?

Possible Answers:

\(\displaystyle -3i - (7-7i) = -7 +4i\)

\(\displaystyle 7i- (3-i) = 3-8i\)

\(\displaystyle 2i - (2-2i) = -2 +4i\)

\(\displaystyle (7-i) -(2+i) = 5-2i\)

\(\displaystyle -(4-i) = (4-i) = 0\)

Correct answer:

\(\displaystyle 7i- (3-i) = 3-8i\)

Explanation:

A problem like this can be solved similarly to a linear binomial like \(\displaystyle (a +bx)\)/

\(\displaystyle 7i - (3-i) = 3-8i\)

\(\displaystyle 7i + (-3 + i) = 3-8i\)

\(\displaystyle -3 +8i \neq 3+8i\)

Example Question #5 : Complex Numbers

Complex numbers take the form \(\displaystyle a+bi\), where \(\displaystyle a\) is the real term in the complex number and \(\displaystyle bi\) is the nonreal (imaginary) term in the complex number.

Which of the following equations simplifies into \(\displaystyle 4+2i\)?

Possible Answers:

\(\displaystyle (8-5i) + (-4 +7i)\)

\(\displaystyle 3 - (7 + 2i)\)

\(\displaystyle 0+ 6i + (-4 - 4i)\)

\(\displaystyle (10 -i) - (6 - i)\)

\(\displaystyle (0 + 7i) - (-4 - 5i)\)

Correct answer:

\(\displaystyle (8-5i) + (-4 +7i)\)

Explanation:

This equation can be solved very similarly to a binomial like \(\displaystyle a+bx\).

\(\displaystyle (8-5i) + (-4 +7i)\)

\(\displaystyle (8-4) + (-5i + 7i)\)

\(\displaystyle 4 + 2i\)

Example Question #2 : Complex Numbers

Suppose \(\displaystyle a=-2\) and \(\displaystyle b=4\)

Evaluate the following expression:

\(\displaystyle 3a^3+\frac{1}{2}(2b^2)\)

Possible Answers:

\(\displaystyle 0\)

\(\displaystyle -8\)

\(\displaystyle 16\)

\(\displaystyle 24\)

\(\displaystyle -24\)

Correct answer:

\(\displaystyle -8\)

Explanation:

Substituting for \(\displaystyle a\) and \(\displaystyle b\), we have

\(\displaystyle 3(-2)^{3}+\frac{1}{2}(2(4)^{2})\)

 This simplifies to

\(\displaystyle -24+16\)

which equals \(\displaystyle -8\)

Example Question #21 : Squaring / Square Roots / Radicals

What is the solution of the following equation?

\(\displaystyle 3\left ( 8 + 5i\right ) + \frac{1}{2}\left (4 + 2i\right ) = ?\)

Possible Answers:

\(\displaystyle 39 + 3i\)

\(\displaystyle 12 + 7i\)

\(\displaystyle 13 + 6i\)

\(\displaystyle 26 + 16i\)

\(\displaystyle 24 +i\)

Correct answer:

\(\displaystyle 26 + 16i\)

Explanation:

A complex number is a combination of a real and imaginary number. To add complex numbers, add each element separately.

First, distribute:

\(\displaystyle 3\left ( 8 + 5i\right ) + \frac{1}{2}\left (4 + 2i\right )\)

\(\displaystyle \left ( 24 + 15i\right ) + \left (2 + i\right )\)

Then, group the real and imaginary components:

\(\displaystyle \left ( 24 + 2\right ) + i\left (15 + 1\right )\)

Solve to get:

\(\displaystyle 26 + 16i\)

Example Question #22 : Squaring / Square Roots / Radicals

What is the sum of \(\displaystyle a\) and \(\displaystyle b\) given

\(\displaystyle a = 5 + 3i\)

and

\(\displaystyle b = 2 + i\)?

Possible Answers:

\(\displaystyle 11\)

\(\displaystyle 3 + 2i\)

\(\displaystyle 8i\)

\(\displaystyle 5 + 6i\)

\(\displaystyle 7 + 4i\)

Correct answer:

\(\displaystyle 7 + 4i\)

Explanation:

A complex number is a combination of a real and imaginary number. To add complex numbers, add each element separately.

In equation \(\displaystyle a\)\(\displaystyle 5\) is the real component and \(\displaystyle 3\) is the imaginary component (designated by \(\displaystyle i\)).

In equation \(\displaystyle b\)\(\displaystyle 2\) is the real component and \(\displaystyle 1\) is the imaginary component.

When added, 

\(\displaystyle a + b = \left ( 5+3i\right ) + \left ( 2 + i\right ) = \left ( 5 + 2\right ) + i\left ( 3 + 1\right ) = 7 + 4i\)

Example Question #9 : Complex Numbers

Complex numbers take the form \(\displaystyle a+bi\), where a is the real term in the complex number and bi is the nonreal (imaginary) term in the complex number.

Simplify: \(\displaystyle (5 +7i) + (17-4i)\)

Possible Answers:

\(\displaystyle 22+3i\)

\(\displaystyle 22 +11i\)

\(\displaystyle 22-3i\)

\(\displaystyle -12-3i\)

\(\displaystyle -12 +3i\)

Correct answer:

\(\displaystyle 22+3i\)

Explanation:

When adding or subtracting complex numbers, the real terms are additive/subtractive, and so are the nonreal terms.

\(\displaystyle (5 +7i) + (17-4i) = (5+17) + (7i - 4i) = 22 + 3i\)

Example Question #10 : Complex Numbers

Complex numbers take the form \(\displaystyle a+bi\), where a is the real term in the complex number and bi is the nonreal (imaginary) term in the complex number.

Can you add the following two numbers: \(\displaystyle 3+7i \textup{ and } 8\)? If so, what is their sum?

 

Possible Answers:

\(\displaystyle \textup{Yes, } 3 + 7i + 8\)

\(\displaystyle \textup{Yes, } 11 + 15i\)

\(\displaystyle \textup{Yes, } 11+ 7i\)

\(\displaystyle \textup{Yes, }3 + 15i\)

\(\displaystyle \textup{No, the two numbers cannot be added because one is complex and one is not.}\)

Correct answer:

\(\displaystyle \textup{Yes, } 11+ 7i\)

Explanation:

Complex numbers take the form a + bi, where a is the real term in the complex number and bi is the nonreal (imaginary) term in the complex number. Taking this, we can see that for the real number 8, we can rewrite the number as \(\displaystyle 8 + 0i\), where \(\displaystyle 0i\) represents the (zero-sum) non-real portion of the complex number.

Thus, any real number can be added to any complex number simply by considering the nonreal portion of the number to be \(\displaystyle 0i\).

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