sin(π/2-a)=sin(π/2)*cosa-cos(π/2)*sina
因为sin(π/2)=1,cos(π/2)=0
所以sin(π/2-a)=1*cosa-0*sina=cosa
cos(π/2-a)=cos(π/2)*cosa+sin(π/2)*sina=0*cosa+1*sina=sina