Question

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

Answer 1

`-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`