GRE Subject Test: Math : Imaginary Numbers & Complex Functions

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

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Example Questions

Example Question #101 : Algebra

Simplify:

\displaystyle \frac{8-3i}{2-i}

Possible Answers:

\displaystyle \frac{19+2i}{5}

\displaystyle 10-7i

\displaystyle 3i+6

\displaystyle \frac{7+2i}{3}

Correct answer:

\displaystyle \frac{19+2i}{5}

Explanation:

To get rid of the fraction, multiply the numerator and denominator by the conjugate of the denominator.

\displaystyle \frac{8-3i}{2-i}\left(\frac{2+i}{2+i}\right)

Now, multiply and simplify.

\displaystyle \frac{8-3i}{2-i}\left(\frac{2+i}{2+i}\right)=\frac{16+8i-6i-3i^2}{4-i^2}

Remember that \displaystyle i^2=-1

\displaystyle \frac{16+8i-6i-3i^2}{4-i^2}=\frac{16+2i-3(-1)}{4-(-1)}=\frac{19+2i}{5}

Example Question #71 : Imaginary Numbers

Divide: \displaystyle \frac{2-i}{3+2i}

The answer must be in standard form.

Possible Answers:

\displaystyle \frac{4}{13}-\frac{7i}{13}

\displaystyle \frac{2}{3}i

\displaystyle i

\displaystyle \frac{2}{3}-\frac{1}{2}i

\displaystyle \frac{1}{5i}

Correct answer:

\displaystyle \frac{4}{13}-\frac{7i}{13}

Explanation:

Multiply both the numerator and the denominator by the conjugate of the denominator which is \displaystyle 3-2i which results in

\displaystyle \frac{\left ( 2-i \right )\left ( 3-2i \right )}{\left ( 3+2i \right )\left ( 3-2i \right )}

The numerator after simplification give us \displaystyle 6-4i-3i+2i^{2}=6-7i+2i^{2}=4-7i

The denominator is equal to \displaystyle 3^{2}-4i^{2}=9+4=13

Hence, the final answer in standard form =

\displaystyle \frac{4}{13}-\frac{7i}{13}

Example Question #41 : Imaginary Numbers & Complex Functions

\displaystyle What\ is\ the\ complex\ conjugate\ for\ 3-i\sqrt{7}?

Possible Answers:

\displaystyle 3+i\sqrt{7}

\displaystyle i\sqrt{7}

\displaystyle -3-i\sqrt{7}

\displaystyle -i\sqrt{7}

Correct answer:

\displaystyle 3+i\sqrt{7}

Explanation:

The definition of a complex conjugate is each of two complex numbers with the same real part and complex portions of opposite sign. 

\displaystyle In\ this\ case\ we\ have\ 3-i\sqrt{7}

\displaystyle to\ find\ its'\ complex\ conjugate\ we\ change\ the\ sign\ of\ the\ complex\ part

\displaystyle The\ complex\ conjugate\ is: 3+i\sqrt{7}

Example Question #42 : Imaginary Numbers & Complex Functions

Which of the following is the complex conjugate of \displaystyle (3+4i)?

Possible Answers:

\displaystyle 1

\displaystyle (3+4i)^2

\displaystyle (3-4i)

\displaystyle (3-4i)^3

\displaystyle 25

Correct answer:

\displaystyle (3-4i)

Explanation:

The complex conjugate of a complex equation \displaystyle (a+bi), \{a,b\}\in \mathbb {R} is \displaystyle (a-bi).

The complex conjugate when multiplied by the original expression will also give me a real answer.

The complex conjugate of \displaystyle (3+4i) is \displaystyle (3-4i)

Example Question #4 : Complex Conjugates

Simplify \displaystyle \frac{14i+4}{7i-2}

Possible Answers:

\displaystyle \frac{20i-23}{24}

\displaystyle \frac{42i+99}{61}

\displaystyle \frac{3i-13}{9}

\displaystyle \frac{-91i+12}{15}

\displaystyle \frac{-90+56i}{-53}

Correct answer:

\displaystyle \frac{-90+56i}{-53}

Explanation:

\displaystyle \frac{14i+4}{7i-2}

In problems like this, you are expected to simplify by removing i from the denominator. To do this, multiply the numerator and denominator by the conjugate of the denominator (switch the sign between the two terms from either a plus to a minus or vice versa) over itself. The conjugate over itself equals 1 and does not change the value of the expression (any number multiplied by 1 is still that number). Multiplying by the conjugate is the only way to eliminate i since there will be no middle term when we foil.

\displaystyle \frac{14i+4}{7i-2}\cdot \frac{7i+2}{7i+2}

\displaystyle \frac{(14i+4)(7i+2)}{(7i-2)(7i+2)}

\displaystyle \frac{98i^{2}+28i+28i+8}{49i^{2}-4}

Simplify i squared to -1 and then combine like terms

\displaystyle \frac{98(-1)+28i+28i+8}{49(-1)-4}

\displaystyle \frac{-98+28i+28i+8}{-49-4}

\displaystyle \frac{-90+56i}{-53}

Example Question #3 : Complex Conjugates

Simplify \displaystyle \frac{2+11i}{3-4i}

Possible Answers:

\displaystyle \frac{-38+41i}{25}

\displaystyle \frac{-9i-28}{11}

\displaystyle \frac{53i+8}{15}

\displaystyle \frac{30i+12}{15}

\displaystyle \frac{17+21i}{19}

Correct answer:

\displaystyle \frac{-38+41i}{25}

Explanation:

\displaystyle \frac{2+11i}{3-4i}

In problems like this, you are expected to simplify by removing i from the denominator. To do this, multiply the numerator and denominator by the conjugate of the denominator (switch the sign between the two terms from either a plus to a minus or vice versa) over itself. The conjugate over itself equals 1 and does not change the value of the expression (any number multiplied by 1 is still that number). Multiplying by the conjugate is the only way to eliminate i since there will be no middle term when we foil.

\displaystyle \frac{2+11i}{3-4i}\cdot \frac{3+4i}{3+4i}

\displaystyle \frac{(2+11i)(3+4i)}{(3-4i)(3+4i)}

\displaystyle \frac{6+33i+8i+44i^{2}}{9-16i^{2}}

Simplify i squared to be -1 and then combine like terms

\displaystyle \frac{6+33i+8i+44(-1)}{9-16(-1)}

\displaystyle \frac{6+33i+8i-44}{9+16}

\displaystyle \frac{-38+41i}{25}

Example Question #2 : Complex Conjugates

Simplify \displaystyle \frac{7i}{-5+4i}

Possible Answers:

\displaystyle \frac{42i-17}{45}

\displaystyle \frac{-11i-24}{9}

\displaystyle \frac{-35i+28}{41}

\displaystyle \frac{22i-14}{17}

\displaystyle \frac{83i+46}{25}

Correct answer:

\displaystyle \frac{-35i+28}{41}

Explanation:

\displaystyle \frac{7i}{-5+4i}

In problems like this, you are expected to simplify by removing i from the denominator. To do this, multiply the numerator and denominator by the conjugate of the denominator (switch the sign between the two terms from either a plus to a minus or vice versa) over itself. The conjugate over itself equals 1 and does not change the value of the expression (any number multiplied by 1 is still that number). Multiplying by the conjugate is the only way to eliminate i since there will be no middle term when we foil.

\displaystyle \frac{7i}{-5+4i}\cdot \frac{-5-4i}{-5-4i}

\displaystyle \frac{7i(-5-4i)}{(-5+4i)(-5-4i)}

\displaystyle \frac{-35i-28i^{2}}{25-16i^{2}}

Simplify i squared to be -1 and then combine like terms

\displaystyle \frac{-35i-28(-1)}{25-16(-1)}

\displaystyle \frac{-35i+28}{25+16}

\displaystyle \frac{-35i+28}{41}

Example Question #4 : Complex Conjugates

Simplify \displaystyle \frac{8i-2}{4i+6}

Possible Answers:

\displaystyle \frac{-16i-19}{13}

\displaystyle \frac{-63i+14}{12}

\displaystyle \frac{-20-56i}{-52}

\displaystyle \frac{32i+19}{34}

\displaystyle \frac{-5-14i}{-13}

Correct answer:

\displaystyle \frac{-5-14i}{-13}

Explanation:

\displaystyle \frac{8i-2}{4i+6}

In problems like this, you are expected to simplify by removing i from the denominator. To do this, multiply the numerator and denominator by the conjugate of the denominator (switch the sign between the two terms from either a plus to a minus or vice versa) over itself. The conjugate over itself equals 1 and does not change the value of the expression (any number multiplied by 1 is still that number). Multiplying by the conjugate is the only way to eliminate i since there will be no middle term when we foil.

\displaystyle \frac{8i-2}{4i+6}\cdot \frac{4i-6}{4i-6}

\displaystyle \frac{(8i-2)(4i-6)}{(4i+6)(4i-6)}

\displaystyle \frac{32i^{2}-8i-48i+12}{16i^{2}-36}

Simplify i squared to be -1 and then combine like terms

\displaystyle \frac{32(-1)-8i-48i+12}{16(-1)-36}

\displaystyle \frac{-32-8i-48i+12}{-16-36}

\displaystyle \frac{-20-56i}{-52}

The coefficients of all the terms can divide by 4 so reduce each of them

\displaystyle \frac{-5-14i}{-13}

Example Question #291 : Gre Subject Test: Math

Simplify \displaystyle \frac{2i}{9i+3}

Possible Answers:

\displaystyle \frac{23i-6}{72}

\displaystyle \frac{49i+27}{52}

\displaystyle \frac{-38+2i}{13}

\displaystyle \frac{-18-6i}{-90}

\displaystyle \frac{3+i}{15}

Correct answer:

\displaystyle \frac{3+i}{15}

Explanation:

\displaystyle \frac{2i}{9i+3}

In problems like this, you are expected to simplify by removing i from the denominator. To do this, multiply the numerator and denominator by the conjugate of the denominator (switch the sign between the two terms from either a plus to a minus or vice versa) over itself. The conjugate over itself equals 1 and does not change the value of the expression (any number multiplied by 1 is still that number). Multiplying by the conjugate is the only way to eliminate i since there will be no middle term when we foil.

\displaystyle \frac{2i}{9i+3}\cdot \frac{9i-3}{9i-3}

\displaystyle \frac{2i(9i-3)}{(9i+3)(9i-3)}

\displaystyle \frac{18i^{2}-6i}{81i^{2}-9}

Simplify i squared to be -1 and then combine like terms

\displaystyle \frac{18(-1)-6i}{81(-1)-9}

\displaystyle \frac{-18-6i}{-81-9}

\displaystyle \frac{-18-6i}{-90}

Since each term divides by a greatest common factor of -6 reduce all of the coefficients. It would also be equivalent to divide by 6 to reduce all of the terms.

\displaystyle \frac{3+i}{15}

Example Question #11 : Complex Conjugates

Simplify \displaystyle \frac{-19i-8}{-5i-2}

Possible Answers:

\displaystyle \frac{-78i+49}{5}

\displaystyle \frac{47+78i}{21}

\displaystyle \frac{-30i+26}{13}

\displaystyle \frac{10i-3}{29}

\displaystyle \frac{-111+2i}{-29}

Correct answer:

\displaystyle \frac{-111+2i}{-29}

Explanation:

\displaystyle \frac{-19i-8}{-5i-2}

In problems like this, you are expected to simplify by removing i from the denominator. To do this, multiply the numerator and denominator by the conjugate of the denominator (switch the sign between the two terms from either a plus to a minus or vice versa) over itself. The conjugate over itself equals 1 and does not change the value of the expression (any number multiplied by 1 is still that number). Multiplying by the conjugate is the only way to eliminate i since there will be no middle term when we foil.

\displaystyle \frac{-19i-8}{-5i-2}\cdot \frac{-5i+2}{-5i+2}

\displaystyle \frac{(-19i-8)(-5i+2)}{(-5i-2)(-5i+2)}

\displaystyle \frac{95i^{2}+40i-38i-16}{25i^{2}-4}

Simplify i squared to be -1 and then combine like terms

\displaystyle \frac{95(-1)+40i-38i-16}{25(-1)-4}

\displaystyle \frac{-95+40i-38i-16}{-25-4}

\displaystyle \frac{-111+2i}{-29}

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