sin(π/4+a)*sin(π/4-a)=1/62*sin(π/4+a)*sin[π/2-(π/4+a)]=2*1/62*sin(π/4+a)*cos(π/4+a)=1/3sin2(π/4+a)=1/3sin(π/2+2a)=1/3cos2a=1/3因为a∈(90,180),所以2a∈(180,360).即sinx取负数sin2a=-√1-cos²...