(1)F(X)=a·b a=(1,-2) b=(1/sin x,cox x) 所以f(x)=1/sin x-2cox x 定义域为x不等于K*PI(PI表示圆周率)K是整数 (2)f(α)=1/sina-2cox a =(1-2(sin a)(cos a))/sin a (通分) =(sin^2(a)+cos^2(a)-2(sin a)(cos a))/sin a (sin^2 a+cos^2 a=1) =(sin a-cos a)^2/sin a tan a=5/12 tan a=sin a/cos a sin^2 a+cos^2 a=1 a为第三象限角sin a