若sin(π/4+a)=1/3,则cos(π/2+2a)的值等于?
问题描述:
若sin(π/4+a)=1/3,则cos(π/2+2a)的值等于?
答
sin(π/4+a)=sina=1/3
cos(π/2+2a)=-cos2a=2*(sina)^2-1=-7/9