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

1 Answer
Jim G.
Mar 29, 2016

-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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