look at a standard sine cureve
sin 270 = -1
so for sin(theta) = -1, theta = 270º or 3pi/4 radians
a general solution would be
theta = (270 + 360k)º where k is any integer
or in radians,
theta = 3pi/4 + 2kpi
Solve sin theta=-1 for all real values of theta
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