use sum to product formula:

[sinx+siny=2sin(x+y)cos(x-y)] to solve [0,2pi).

sin3x + sinx = 0

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this is what i did and got stuck on:

1. sin(3x+x)

2. (2sin(3x+x)/2)cos(3x-x)/2

3. 2sin(4x/2)cos(2x/2)

4. 2sin2xcosx

stuck there, what do i do next?