How do you express $cos((27pi)/8)$ as a trig function of an angle in Quadrant I?

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Answer 1 · 2017-02-28T03:23:21

$- cos ((3pi)/8)$

On the unit circle,
$cos ((27pi)/8) = cos ((11pi)/8 + 2pi) = cos ((11pi)/8) =$
$= cos ((3pi)/8 + pi) = - cos ((3pi)/8)$

How do you express cos((27pi)/8)cos((27pi)/8) as a trig function of an angle in Quadrant I?