f(x)=sin(2x+π/3)变不成cos2x呀
可以变成cos(2x+φ)形式
f(x)=sin(2x+π/3)
=sin[2x+(π/2-π/6)]
=sin[π/2+(2x-π/6)] 【诱导公式：sin(π/2+α)=cosα】
=cos(2x-π/6)