求cos(π/2+α)sin(-π-α) / cos(11π/2 -α)sin(9π/2+α)的值
问题描述：
求cos(π/2+α)sin(-π-α) / cos(11π/2 -α)sin(9π/2+α)的值
答
cos(π/2+α)=-sinα,因为若α在第一象限,π/2+α在第二,cos是负值 (换成sin因为差的是π/2的倍数)sin(-π-α)=sin(-π+2π-α) =sin(π-α)=sinα,α在第一象限,π-α在第二,sin是正值cos(11π/2 -α)=cos(11π/2-4...