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

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Alan P. Share
Jun 1, 2016

Answer:

#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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