sinx=2cosx,sin^2x+2sinxcosx=sin^2x+2sinx*1/2sinx=2sin^2x
sin^2x+cos^2x=1 sin^2x+(1/2 sinx)^2=1 5/4 sin^2x=1 sin^2x=4/5 所以原式等于2*4/5=8/5