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

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Jim G. Share
Mar 29, 2016

Answer:

# -1/sqrt2#

Explanation:

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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