ผมทำได้แบบนี้ครับ
$\sqrt{\frac{x^3}{x^3+(y+z)^3}}=\sqrt{\frac{1}{1 +(\frac{y+z}{x})^3}} \ge \frac{2x^2}{(y+z)^2+2x^2}$
AM-GM อีกรอบ
$\frac{2x^2}{(y+z)^2+2x^2} \ge \frac{x^2}{x^2+y^2+z^2}$
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