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How do you evaluate $cos (\frac{9 π}{4})$ $cos ((9pi)/4)$ ?

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Apr 16, 2016

$= cos (2 π + \frac{π}{4}) = cos (\frac{π}{4}) = \frac{1}{\sqrt{2}}$ $=cos(2pi+pi/4)=cos (pi/4)=1/sqrt2$

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