Lời giải:
Đặt $\frac{y}{x}=a(a>0)$ thì:
\(P=\sqrt{\frac{1}{1+(\frac{2y}{x})^3}}+\sqrt{\frac{4}{1+(1+\frac{x}{y})^3}}=\sqrt{\frac{1}{1+8a^3}}+\sqrt{\frac{4}{1+(1+\frac{1}{a})^3}}\)
Áp dụng BĐT AM-GM dạng $xy\leq \left(\frac{x+y}{2}\right)^2$ ta có:
\(1+8a^3=1+(2a)^3=(1+2a)(1-2a+4a^2)\leq \left(\frac{1+2a+1-2a+4a^2}{2}\right)^2=(2a^2+1)^2\)
\(\Rightarrow \sqrt{\frac{1}{8a^3+1}}\geq \frac{1}{2a^2+1}(1)\)
\(1+(1+\frac{1}{a})^3=(2+\frac{1}{a})[1-(1+\frac{1}{a})+(1+\frac{1}{a})^2]\leq (\frac{3a^2+2a+1}{2a^2})^2\)
\(\Rightarrow \sqrt{\frac{4}{1+(1+\frac{1}{a})^3}}\geq \frac{4a^2}{3a^2+2a+1}\)
Mà: \(\frac{4a^2}{3a^2+2a+1}\geq \frac{4a^2}{3a^2+a^2+1+1}=\frac{2a^2}{2a^2+1}\) nên \(\sqrt{\frac{4}{1+(1+\frac{1}{a})^3}}\geq \frac{2a^2}{2a^2+1}(2)\)
Từ $(1);(2)\Rightarrow P\geq \frac{1}{2a^2+1}+\frac{2a^2}{2a^2+1}=1$
Vậy $P_{\min}=1$ khi $a=1\Leftrightarrow x=y$
