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GRE Subject Test: Math : Complex Conjugates

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

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Example Questions

Example Question #291 : Gre Subject Test: Math

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

Possible Answers:

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

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

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

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

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

Correct answer:

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

Explanation:

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

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.

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

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

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

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

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

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

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

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.

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

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Example Question #43 : Imaginary Numbers & Complex Functions

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

Possible Answers:

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

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

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

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

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

Correct answer:

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

Explanation:

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

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

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

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

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

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

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

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

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Example Question #292 : Gre Subject Test: Math

Simplify $\frac{i-2}{3-i}$

Possible Answers:

$\frac{2i-5}{5}$

$\frac{10i-2}{7}$

$\frac{i-7}{10}$

$\frac{7-i}{10}$

$\frac{5-2i}{10}$

Correct answer:

$\frac{i-7}{10}$

Explanation:

$\frac{i-2}{3-i}$

$\frac{i-2}{3-i}\cdot \frac{3+i}{3+i}$

$\frac{(i-2)(3+i)}{(3-i)(3+i)}$

$\frac{3i+i^{2}-6-2i}{9-i^{2}}$

Simplify i squared to be -1 and combine like terms

$\frac{3i+(-1)-6-2i}{9-(-1)}$

$\frac{3i-1-6-2i}{9+1}$

$\frac{i-7}{10}$

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Example Question #293 : Gre Subject Test: Math

Simplify $\frac{14i-9}{9+i}$

Possible Answers:

$\frac{54i-18}{25}$

$\frac{23i-11}{65}$

$\frac{-44i+63}{86}$

$\frac{135i-67}{82}$

$\frac{14i+111}{42}$

Correct answer:

$\frac{135i-67}{82}$

Explanation:

$\frac{14i-9}{9+i}$

$\frac{14i-9}{9+i}\cdot \frac{9-i}{9-i}$

$\frac{(14i-9)(9-i)}{(9+i)(9-i)}$

$\frac{126i-81-14i^{2}+9i}{81-i^{2}}$

Simplify i squared to be -1 and combine like terms

$\frac{126i-81-14(-1)+9i}{81-(-1)}$

$\frac{126i-81+14+9i}{81+1}$

$\frac{135i-67}{82}$

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Example Question #294 : Gre Subject Test: Math

Simplify $\frac{13i+13}{13i-9}$

Possible Answers:

$\frac{-13+169i}{250}$

$\frac{26-143i}{250}$

$\frac{-13i-169}{-250}$

$\frac{-26+143i}{-125}$

$\frac{26i-143}{125}$

Correct answer:

$\frac{-26+143i}{-125}$

Explanation:

$\frac{13i+13}{13i-9}$

$\frac{13i+13}{13i-9}\cdot \frac{13i+9}{13i+9}$

$\frac{(13i+13)(13i+9)}{(13i-9)(13i+9)}$

$\frac{(169i^{2}+169i+117i+117)}{(169i^{2}-81)}$

Simplify i squared to be -1 and combine like terms

$\frac{(169(-1)+169i+117i+117)}{(169)(-1)-81}$

$\frac{(-169+169i+117i+117)}{(-169-81)}$

$\frac{-52+286i}{-250}$

Each term divides by 2 so make sure to reduce all of the terms

$\frac{-26+143i}{-125}$

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Example Question #295 : Gre Subject Test: Math

Simplify $\frac{-12i-17}{6i+11}$

Possible Answers:

$\frac{-72i+13}{99}$

$\frac{24i-15}{81}$

$\frac{-20i-187}{34}$

$\frac{33i-105}{16}$

$\frac{19i+259}{-157}$

Correct answer:

$\frac{19i+259}{-157}$

Explanation:

$\frac{-12i-17}{6i+11}$

$\frac{-12i-17}{6i+11}\cdot \frac{6i-11}{6i-11}$

$\frac{(-12i-17)(6i-11)}{(6i+11)(6i-11)}$

$\frac{-72i^{2}+121i-102i+187}{36i^{2}-121}$

Simplify i squared to be -1 and combine like terms

$\frac{-72(-1)+121i-102i+187}{36(-1)-121}$

$\frac{72+121i-102i+187}{-36-121}$

$\frac{19i+259}{-157}$

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Example Question #51 : Imaginary Numbers & Complex Functions

Simplify $\frac{3i+7}{4i-4}$

Possible Answers:

Undefined

$\frac{4i+9}{20}$

$\frac{11i-2}{6}$

$\frac{6i+7}{18}$

$\frac{5i+2}{-4}$

Correct answer:

$\frac{5i+2}{-4}$

Explanation:

$\frac{3i+7}{4i-4}$

$\frac{3i+7}{4i-4}\cdot \frac{4i+4}{4i+4}$

$\frac{(3i+7)(4i+4)}{(4i-4)(4i+4)}$

$\frac{12i^{2}+12i+28i+28}{16i^{2}-16}$

Simplify i squared to be -1 and combine like terms

$\frac{12(-1)+12i+28i+28}{16(-1)-16}$

$\frac{-12+12i+28i+28}{-16-16}$

$\frac{40i+16}{-32}$

Since every term divides by 8 make sure to reduce all the terms by that greatest common factor

$\frac{5i+2}{-4}$

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Example Question #52 : Imaginary Numbers & Complex Functions

Simplify

$\frac{5+2i}{-3-2i}$

Possible Answers:

6+i

$\frac{7i+1}{5}$

$\frac{i+2}{10}$

$\frac{-19+4i}{13}$

$\frac{16+4i}{3}$

Correct answer:

$\frac{-19+4i}{13}$

Explanation:

$\frac{5+2i}{-3-2i}$

$\frac{5+2i}{-3-2i}\cdot \frac{-3+2i}{-3+2i}$

$\frac{(5+2i)(-3+2i)}{(-3-2i)(-3+2i)}$

$\frac{-15-6i+10i+4i^{2}}{9-4i^{2}}$

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

$\frac{-15-6i+10i+4(-1)}{9-4(-1)}$

$\frac{-15+4i-4}{9+4}$

$\frac{-19+4i}{13}$

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GRE Subject Test: Math : Complex Conjugates

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

All GRE Subject Test: Math Resources

Example Questions

Example Question #291 : Gre Subject Test: Math

Example Question #43 : Imaginary Numbers & Complex Functions

Example Question #292 : Gre Subject Test: Math

Example Question #293 : Gre Subject Test: Math

Example Question #294 : Gre Subject Test: Math

Example Question #295 : Gre Subject Test: Math

Example Question #51 : Imaginary Numbers & Complex Functions

Example Question #52 : Imaginary Numbers & Complex Functions

All GRE Subject Test: Math Resources

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