# Hi, I'm in need of some help understanding some of my precalculus homework. Suppose θ is an angle such that cos θ = 3/4. Evaluate cos(2θ).?

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- PhilipLv 62 months ago
Replace theta by x.

cos(x) = (3/4);

cos(2x) = 2cos^2(x) - 1 = 2(9/16) - 1 = 18/16 -1 = 2/16 = 1/8.

- ?Lv 72 months ago
cos θ = 3/4

cos 2θ = 2 cos² θ -1

............= 2 (3/4)² - 1

............= 2 (9/16) - 1

............= 18/32 - 1

............= -14/32

............= -7/16..............ANS

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- ♥Astrid♥Lv 72 months ago
Trig Identity: cos(2x) = 2cos²(x) – 1

Given: cos(θ) = 3/4

cos(2θ) = 2cos²(θ) - 1

cos(2θ) = 2(3/4)² - 1

cos(2θ) = 2(9/16) - 1

cos(2θ) = 1/8

- oldprofLv 72 months ago
ACOS(3/4) = 41.4126724 deg so that 2theta = 82.82 and cos(82.82) = 0.124986913 ANS.

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