sin(x+π/3)=sinxcos(π/3)+cosxsin(π/3)=1/2sinx+√3/2cosx
2sin(x-π/3)=2sinxcos(π/3)-2cosxsin(π/3)=sinx+√3cosx
-√3cos(2π/3-x)=-√3(cos(2π/3)cosx+sin(2π/3)sinx)=-√3(-1/2cosx+√3/2sinx)=√3/2cosx-3/2xsinx
把上面三个式子加起来就是2√3cosx
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