Math Question Multiple Choice

0

462

1

Find cos2A when cosA = -3/5 and π/2<A<π.

A) -24/25

B) -7/25

C) 7/24

D) 24/7

Guest Oct 20, 2014

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Best Answer

+128707

+5

cos2A = cos^2(A) - sin^2(A)

So, if cos(A) = -3/5......then

1 - cos^2(A) = sin^(A)

1 - (9/25) = sin^2(A)

16/25 = sin^2(A)

So

cos(2A) = (9/25) - (16/25) = -7/25

CPhill Oct 20, 2014

1⁺⁰ Answers

+128707

+5

Best Answer

cos2A = cos^2(A) - sin^2(A)

So, if cos(A) = -3/5......then

1 - cos^2(A) = sin^(A)

1 - (9/25) = sin^2(A)

16/25 = sin^2(A)

So

cos(2A) = (9/25) - (16/25) = -7/25

CPhill Oct 20, 2014

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