sinx+sin(x+2π/3)+sin(x+4π/3)
=sinx+sin(x+2π/3)+sin(x+2π/3)cos(2π/3)+sin(2π/3)cos(x+2π/3)
=sinx+sin(x+2π/3)-(1/2)sin(x+2π/3)+(√3/2)cos(x+2π/3)
=sinx+(1/2)sin(x+2π/3)+(√3/2)cos(x+2π/3)
=sinx+sin(x+2π/3+π/3)
=sinx+sin(x+π)
=sinx-sinx
=0
sinx+sinxcos120+cosxsin120+sinxcos240+cosxsin240
=sinx-1/2sinx+3^1/2/2cosx-1/2sinx-3^1/2/2cosx
=0
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