GRE Subject Test: Math : Operations on Complex Numbers

Study concepts, example questions & explanations for GRE Subject Test: Math

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Example Questions

Example Question #1 : Operations On Complex Numbers

Expand and Simplify: \(\displaystyle (4-3i)(2+3i)(2-3i)\)

Possible Answers:

\(\displaystyle 39+52i\)

\(\displaystyle -20-15i\)

\(\displaystyle 52-39i\)

\(\displaystyle 52+39i\)

Correct answer:

\(\displaystyle 52-39i\)

Explanation:

Step 1: We will multiply the two complex conjugates: \(\displaystyle 2x-3\) and \(\displaystyle 2x+3\).

\(\displaystyle (2-3i)(2+3i)=4+6i-6i-9i^2=4-9i^2\)

Step 2: Replace \(\displaystyle i^2\) with \(\displaystyle -1\).

\(\displaystyle 4-9(-1)\)

Simplify:

\(\displaystyle 4+9(1)\)
\(\displaystyle =13\)


Step 3: Multiply the result of the complex conjugates to the other parentheses,\(\displaystyle (4-3i)\).

\(\displaystyle 13(4-3i)=52-39i\)

The final answer after the product of all three binomials is \(\displaystyle 52-39i\)
 

Example Question #1 : Operations On Complex Numbers

Expand: \(\displaystyle (i-1)^{10}\).

Possible Answers:

\(\displaystyle -32i\)

\(\displaystyle 32i\)

\(\displaystyle 32(1-i)\)

\(\displaystyle -i\)

Correct answer:

\(\displaystyle -32i\)

Explanation:

Quick Way:
Step 1: Expand \(\displaystyle (i-1)^2\) .

\(\displaystyle (i-1)^2=i^2-2i+1\).

Remember: \(\displaystyle i^2=-1\)

\(\displaystyle (i-1)^2=-1-2i+1=-2i\)

Step 2: \(\displaystyle (1-i)^{10}=(i-1)^{2\cdot 5}\)

By this equivalence, I can just raise the answer of \(\displaystyle (1-i)^2\) to the power \(\displaystyle 5\).

\(\displaystyle (i-1)^{10}=(-2i)^{5}=-32i^5\)

\(\displaystyle i^5 \equiv i^1=i\). Replace \(\displaystyle i^5\)..

Final answer: \(\displaystyle -32i\)

Long Way:



Math work

Example Question #1 : Operations On Complex Numbers

Multiply: 

\(\displaystyle (2+3i)(3-4i)\)

Possible Answers:

\(\displaystyle 6+i\)

\(\displaystyle 18+i\)

\(\displaystyle 18-i\)

\(\displaystyle -i-18\)

Correct answer:

\(\displaystyle 18+i\)

Explanation:

Step 1: FOIL:

Recall, FOIL means to multiply the first terms in both binomials together, the outer terms together, the inner terms together, and finally, the last terms together.

\(\displaystyle (2+3i)(3-4i)=6-8i+9i-12i^2\)

Step 2: Simplify:

\(\displaystyle 6+i-12i^2\)

Step 3: Recall: \(\displaystyle i^2=-1\). Replace and simplify.

\(\displaystyle 6+i-12(-1)\)
\(\displaystyle 6+i+12\)
\(\displaystyle =18+i\)

Example Question #1 : Operations On Complex Numbers

\(\displaystyle Simplify: (3-5i)+(2+7i)\)

Possible Answers:

\(\displaystyle 7i\)

\(\displaystyle 5+2i\)

\(\displaystyle 41+11i\)

\(\displaystyle 6-35i\)

Correct answer:

\(\displaystyle 5+2i\)

Explanation:

When adding imaginary numbers, simply add the real parts and the imaginary parts. 

\(\displaystyle (3-5i)+(2+7i)= (3+2)+(-5i+7i)=5+2i\)

Example Question #2 : Operations On Complex Numbers

\(\displaystyle Simplify: (13-7i)-(10-8i)\)

 

Possible Answers:

\(\displaystyle 23-i\)

\(\displaystyle 3+i\)

\(\displaystyle 3-15i\)

\(\displaystyle -74-174i\)

Correct answer:

\(\displaystyle 3+i\)

Explanation:

\(\displaystyle When\ subtracting\ binomials\ in\ ( )\; first\ distribute\ the\ negative\ sign.\)

\(\displaystyle (13-7i)-(10-8i)=13-7i-10+8i\)

\(\displaystyle Now\ combine\ like\ terms.\)

\(\displaystyle 13-10-7i+8i=3+i\)

Example Question #2 : Operations On Complex Numbers

What is the value of \(\displaystyle (3-4i)(2i+3)\)?

Possible Answers:

\(\displaystyle -6i-17\)

None of the other answers

\(\displaystyle -6i+17\)

\(\displaystyle -6i+1\)

\(\displaystyle 6i-17\)

Correct answer:

\(\displaystyle -6i+17\)

Explanation:

Distribute and Multiply:

\(\displaystyle (3-4i)(2i+3)=6i+9-8i^2-12i\)

Simplify all terms...

\(\displaystyle 6i+9-8(-1)-12i\)

\(\displaystyle =6i+9+8-12i\)

\(\displaystyle =-6i+17\)

Example Question #5 : Operations On Complex Numbers

What is the value: \(\displaystyle i^{24}(i^{3}+i^{-2})\)?

Possible Answers:

\(\displaystyle -i-1\)

\(\displaystyle 1+i\)

\(\displaystyle -1-i\)

\(\displaystyle -(-i+1)\)

Correct answer:

\(\displaystyle -1-i\)

Explanation:

Step 1: Recall the cycle of imaginary numbers to a random power \(\displaystyle x\).

If \(\displaystyle x=1\), then \(\displaystyle i^1=i\)

If \(\displaystyle x=2\), then \(\displaystyle i^2=-1\)

If \(\displaystyle x=3\), then \(\displaystyle i^3=-i\)

If \(\displaystyle x=4\), then \(\displaystyle i^4=1\)

If \(\displaystyle x=5\), then \(\displaystyle i^5=i^4(i^1)=1(i)=i\)

and so on....

The cycle repeats every \(\displaystyle 4\) terms. 

For ANY number \(\displaystyle x\ge 5\), you can break down that term into smaller elementary powers of i. 

Step 2: Distribute the \(\displaystyle i^{24}\) to all terms in the parentheses:

\(\displaystyle (i^{24})(i^3)+(i^{24})(i^{-2})\).

Step 3: Recall the rules for exponents:

\(\displaystyle \forall\{a,b,c,d\} \in \mathbb{R}::\)

\(\displaystyle (a^b)\cdot (a^c)=a^{b+c}\)

\(\displaystyle {a^{-b}}=\frac {1}{a^b}\)

\(\displaystyle (a^b)\cdot (a^{-c})=a^b \cdot \bigg (\frac {1}{a^c}\bigg)=\frac {a^b}{a^c}=a^{b-c}=a^d\)

Step 4: Use the rules to rewrite the expression in Step 2:

\(\displaystyle (i^{24})\cdot {i^3}=i^{24+3}=i^{27}\)

\(\displaystyle (i^{24})(i^{-2})=\frac {i^{24}}{i^2}=i^{24-2}=i^{22}\)

Step 5: Simplify the results in Step 4. Use the rules in Step 1.:

\(\displaystyle i^{27}=(i^{24}\cdot i^3)=1(-i)=-i\)

\(\displaystyle i^{22}=(i^{20}\cdot i^{2})=1(-1)=-1\)

Step 6: Write the answer in \(\displaystyle a+bi\) form, where \(\displaystyle a\) is the real part and \(\displaystyle bi\) is the imaginary part:

We get \(\displaystyle -1-i\)

Example Question #3 : Operations On Complex Numbers

\(\displaystyle Add: (3-5i)+(2+7i)\)

Possible Answers:

\(\displaystyle 1-12i\)

\(\displaystyle 1+12i\)

\(\displaystyle 5+2i\)

\(\displaystyle 6-35i\)

Correct answer:

\(\displaystyle 5+2i\)

Explanation:

When adding complex numbers, we add the real numbers and add the imaginary numbers. 

\(\displaystyle Add: (3-5i)+(2+7i)\)

\(\displaystyle =(3+2)+(-5i+7i)\)

\(\displaystyle =5+2i\)

 

 

Example Question #4 : Operations On Complex Numbers

\(\displaystyle Subtract: (15+7i)-(12+9i)\)

Possible Answers:

\(\displaystyle 3-2i\)

\(\displaystyle 3+16i\)

\(\displaystyle 3-5i\)

\(\displaystyle 3-16i\)

Correct answer:

\(\displaystyle 3-2i\)

Explanation:

In order to subtract complex numbers, we must first distribute the negative sign to the second complex number. 

\(\displaystyle Subtract: (15+7i)-(12+9i)\)

\(\displaystyle 15+7i-12-9i\)

\(\displaystyle 15-12+7i-9i=3-2i\)

 

Example Question #4 : Operations On Complex Numbers

\(\displaystyle Simplify: 3i(-5+2i)\)

Possible Answers:

\(\displaystyle 15+6i^2\)

\(\displaystyle -6-15i\)

\(\displaystyle 15-6i\)

\(\displaystyle 6i^2-15i\)

Correct answer:

\(\displaystyle -6-15i\)

Explanation:

\(\displaystyle Simplify: 3i(-5+2i)\)

First we must distribute

\(\displaystyle 3i(-5+2i)=-15i+2i^2\)

\(\displaystyle We\ must\ remember\ that\ i^2=-1\)

\(\displaystyle -15i+6i^2=-15i+6(-1)\)

\(\displaystyle -15i+6(-1)=-6-15i\ (i\ term\ is\ always\ last)\)

 

 

 

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