Solve the equation over the interval [0,2pi)
-sinx = cosx
-sin x = cos x rearrange as
cosx + sin x = 0
Note :
at 3pi/4 sin = sqrt (2) /2 and cos = -sqrt (2) /2
at 7pi/4 sin = -sqrt (2) /2 and cos = sqrt (2) / 2
So ... 3pi/4 and 7pi /4