Question

what is the value of cos 135

Answer 1

cos 135 =cos(180-45) = -cos 45 = -1/√2
here, we get negative sign since the value of cos in 2nd quadrant is negative. since we change cos from 180, it remains as cos only without changing into sin

Answer 2

Cos 135 degrees = cos (90 +45) = - sin45 = - 1/√2