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How do you find the exact value of cos 5pi/4?

1 Answer

Jun 1, 2016

$cos((5pi)/4)= -1/sqrt(2) or -sqrt(2)/2$

Explanation:

$(5pi)/4$ is an angle in Quadrant III and as such (based on CAST) its $cos$ is negative.

$(5pi)/4=pi+pi/4$
So its reference angle is $pi/4$ which is a standard angle with $cos(pi/4)=1/sqrt(2)$

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