y=—cos2x=sin(2x+3π/2)为什么?

问题描述:

y=—cos2x=sin(2x+3π/2)为什么?

sin(2x+3π/2)=sin2xcos3π/2+cos2xsin3π/2
cos3π/2=0,sin3π/2=-1
原式=-cos2x