回復 39# weiye 的帖子
計算 2. 延續 weiye 老師的體積結果 \( V = \displaystyle \frac{2\pi a^{2}}{15(1-a)^{\frac52}} \)
令 \( a = -\tan^2 \theta \),則 \( V = \frac{2\pi}{15}\sin^4\theta\cos\theta \)
由算幾不等式有 \(\displaystyle 1=\frac{\sin^{2}\theta}{4}+\frac{\sin^{2}\theta}{4}+\frac{\sin^{2}\theta}{4}+\frac{\sin^{2}\theta}{4}+\cos^{2}\theta\geq5\sqrt[5]{\frac{\sin^{8}\theta\cos^2\theta}{256}} \),
(感謝眼尖的 wooden 挑出筆誤,已修正上行)
且當 \( \tan^{2}\theta=4 \) 時,等式成立,故 \( a=-4 \) 時有最大值 \(\displaystyle \frac{32\pi}{375\sqrt{5}} \)。