已知sina=1/3,则sin(a+π/4)cos(a+π/4)

问题描述:

已知sina=1/3,则sin(a+π/4)cos(a+π/4)

sin(a+π/4)cos(a+π/4)
=(1/2)*sin[2(a+π/4)]
=(1/2)*sin(2a+π/2)
=(1/2)*cos(2a)
=(1/2)*[1-2*(sina)^2]
=(1/2)*[1-2*(1/3)^2]
=7/18