How do you evaluate $cos ((5pi)/3)$ ?

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How do you evaluate $cos ((5pi)/3)$ ?

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Answer 1 · 2016-03-18T00:55:23

$1/2$

Explanation:

Apply the trig identity: cos 2a = 2cos^2 a - 1
$cos ((5pi)/3) = 2cos^2 ((5pi)/6) - 1.$
Trig table gives --> $cos ((5pi)/6) = -sqrt3/2$
Therefor,
$cos ((5pi)/3) = 2(3/4) - 1 = 3/2 - 1 = 1/2$

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How do you evaluate $cos ((5pi)/3)$ ?

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