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?

sin(π/4+a)=sina=1/3
cos(π/2+2a)=-cos2a=2*(sina)^2-1=-7/9