How do I find the value of cos 11pi / 6?

1 Answer
Nghi N.
Aug 14, 2015

Find cos ((11pi)/6)

Ans: sqrt3/2

Explanation:

Cal cos ((11pi)/6) = cos t

cos 2t = cos ((22pi)/6) = cos ((-2pi)/6 + 12(2pi)) =
= cos (-pi/3) = cos (pi/3) = 1/2
Use trig identity: cos 2t = 2cos^2 t - 1

2cos^2 t = 1 + 1/2 = 3/2
cos^2 t = 3/4
cos ((11pi)/6) = cos t = +- sqrt3/2
Since the arc (11pi)/6 is located in Quadrant IV, only the positive answer is accepted.
cos ((11pi)/6) = sqrt3/2

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