How do you evaluate $cos 2(pi/3)$ ?

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Answer 1 · 2016-02-17T12:48:10

$cos2(pi/3)=-1$

$cos 2(pi/3)=cos((2pi)/3)$

$(2pi)/3$ is in quadrant Q II and has a reference angle of $pi/3$

$pi/3$ is a standard angle with $cos(pi/3)=1/2$
but since $(2pi)/3$ is in Q II
$cos((2pi)/3)=-cos(pi/3) =-1/2$
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How do you evaluate cos 2(pi/3)cos 2(pi/3)?