note that 5π /4 is in the third quadrant where the cos is negative
the related acute angle to 5π /4 is π/4, thus
cos( 5π /4 ) = - cos (π/4 ) = - √2/2
We can also evaluate using the addition formula for cosine
• cos (x + y ) = cosxcosy - sinxsiny
note that 5π /4 = (π + π/4 )
cos(5π /4 ) = cos (π + π/4 )
= cos(π)cos(π/4) - sin(π)sin(π/4)
= - 1 √2/2 - 0 = - √2/2