1.微分法
2.算幾不等式
考慮(sinθ)^4*(cosθ)^2的範圍
由(sinθ)^2+(cosθ)^2=1 得 (1/2)(sinθ)^2+(1/2)(sinθ)^2+(cosθ)^2=1
(1/2)(sinθ)^2+(1/2)(sinθ)^2+(cosθ)^2≧3[(1/4)(sinθ)^4*(cosθ)^2]^(1/3)
=> (sinθ)^4*(cosθ)^2≦4/27
=>-2√3/9≦(sinθ)^2*(cosθ)≦2√3/9
等號成立於(1/2)(sinθ)^2=(cosθ)^2=1/3
即cosθ=√3/3,θ= ±cos^(-1)√3/3 +2kπ時,最大值2√3/9