sin 2x= 2 sin x
2 sin x cos x= 2sin x
2 sin x cos x - 2 sin x=0
2 sin x (cos x -1)=0
It's a two factors multiplication, and the result is 0 when one of these two factors is 0, too.
2 sin x =0, sin x=0
x=arcsin 0
x=0 or x=pi
cos x - 1=0
cos x=1
x=+/-arccos 1
x=0, x=pi
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