cos2a=cos(π/4－a)sin[π/2－2a]=cos(π/4－a)sin[2(π/4－a)]=cos(π/4－a)2sin(π/4－a)cos(π/4－a)=cos(π/4－a)则：sin(π/4－a)=1/2即：(√2/2)sina－(√2/2)cosa=1/2sina－cosa=√2/2(sina－cosa)²=1/21－2sinacosa=1/2sin2a=1/2
