y=cosθ-sin^2θ+1 =cosθ-(1-cos^2θ)+1 =cosθ+cos^2θ =[cosθ+1/2]^2-1/4-1<=cosθ<=1所以y最大值为y=[1+1/2]^2-1/4=2最小值为y==[-1/2+1/2]^2-1/4=-1/4
把 sin 换成1-cos^2然后换元成二次函数