Answer :

AL2006
cos (X) + sin (X) = 0

Subtract cos(x) from each side:

sin(x) = -cos(x)

Divide each side by cos(x) :

sin(x) / cos(x) = -1

But sin/cos = tangent

tan(x) = -1

x = 135° and 315°

x = 3/4 pi radians  and  7/4 pi radians

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