sinα/√(1+cot^2)-cosα/√(1+tan^2)=-1sina/|csca|-cosa/|seca|=-1一象限：左边=sin^2a-cos^2a=-cos2a=-1,a=0,不在定义域内二象限：左边=sin^2a+cos^2a=1≠-1三象限：左边=-sin^2a+cos^2a=cos2a=-1,a=π/2,不在定...