use the identity that
sin²(x)+cos²(x)=1
so
cos²(x)=1+sin(x)
cos²(x)=sin²(x)+cos²(x)+sin(x)
minus cos²(x) both sides
0=sin²(x)+sin(x)
thefor
sin²(x)=-sin(x)
only possible if sin(x)=0
or if sin(x)=-1
that occurs at x=0 and x=3pi/2
but parethasees so 0 is not included
x=3pi/2