How do you evaluate $cos^2(pi/8) - sin^2(pi/8)$ ?

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Answer 1 · 2016-04-07T02:49:15

$sqrt2/2$

Apply the trig identity: $cos 2a = cos^2 a - sin^2 a$
$cos^2 (pi/8) - sin^2 (pi/8) = cos (pi/4)$
Trig table --> $cos (pi/4) = sqrt2/2$

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