✔ 最佳答案
y^2=(cosθ)^2*(1+sinθ)^2=(1-sinθ)(1+sinθ)^3
又 (1+sinθ)+(1-sinθ)=2, or (1+sinθ)/3+(1+sinθ)/3+(1+sinθ)/3 +(1-sinθ)=2
由算幾不等式: (2/4)^4 >= [(1+sinθ)/3]^3 *(1-sinθ)
得y^2= (1-sinθ)(1+sinθ)^3 <= 27/16
故 -3√3/4 <= y <= 3√3/4
最大值=3√3/4, 最小值=-3√3/4
註: 1-sinθ, 1+sinθ>=0, 故可用算幾不等式
(1-sinθ)(1+sinθ)^3最大時, 1-sinθ=(1+sinθ)/3, sinθ=1/2 (可)