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2 votes
Find an exact value.


cos(- ( \77 \pi )/(12))

User Sergino
by
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1 Answer

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\cos\left(-(7\pi)/(12)\right)=\cos(7\pi)/(12)

Recall that


\cos^2x=\frac{1+\cos2x}2

Let
x=(7\pi)/(12), so that


\cos^2(7\pi)/(12)=\frac{1+\cos\frac{7\pi}6}2=\frac{1-\frac{\sqrt3}2}2=\frac{2-\sqrt3}4

\cos(7\pi)/(12)=\pm\sqrt{\frac{2-\sqrt3}4}=\pm\frac{√(2-\sqrt3)}2

Since the angle
(7\pi)/(12) lies in the second quadrant, you know that the cosine must be negative, so


\cos(7\pi)/(12)=-\frac{√(2-\sqrt3)}2
User Sgwill
by
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