sin(π/4+a)=cos[π/2-(π/4+a)]=cos(π/4-a)所以分母=2[sin(π/4-a)/cos(π/4-a)]*cos²(π/4-a)=2sin(π/4-a)cos(π/4-a)=sin[2(π/4-a)]=sin(π/2-2a)=cos2a分子=cos2a所以原式=1