Điều kiện \(a>0\)
\(A=\sqrt[4]{\frac{3}{4a}}.\sqrt[4]{\frac{4a}{3}}.x\sqrt{a-x^4}\le\sqrt[4]{\frac{3}{4a}}\left(-x^4+\sqrt{\frac{4a}{3}}x^2+a\right)\)
\(A\le\sqrt[4]{\frac{3}{4a}}\left[\frac{4a}{3}-\left(x^2-\sqrt{\frac{a}{3}}\right)^2\right]\le\frac{4a}{3}\sqrt[4]{\frac{3}{4a}}\)
Dấu "=" xảy ra khi \(x=\sqrt[4]{\frac{a}{3}}\)