# How do you find the exact value of cos 3pi/4?

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Jim G.
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Mar 29, 2016

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The angle

# (3pi)/4 " is in the 2nd quadrant"# where the cos ratio has a negative value. Now the related acute angle for

#(3pi)/4 " is " pi/4 # then

# cos((3pi)/4) = - cos(pi/4) # Using the 45-45-90 degree triangle with sides 1 , 1 ,

#sqrt2 # where

# cos45^@ = cos(pi/4) = 1/sqrt2 #

#rArr cos((3pi)/4) = - cos(pi/4) = - 1/sqrt2 #

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