Đặt $x+y=2t \Rightarrow 1=x^{3}+y^{3}=(x+y)^3-3xy(x+y) \Rightarrow 1=8t^{3}-6xyt \Rightarrow t^{2}=(\frac{x+y}{2})^2\geq xy=\frac{8t^{3}-1}{6t}$ $\Rightarrow 0< t\leq \frac{1}{\sqrt[3]{2}} $
Ta có :
$A=\frac{x^{2}+y^{2}}{(1-x)(1-y)}=\frac{2(x+y)^2+2(x-y)^2}{((1-x)+(1-y))^2-((1-x)-(1-y))^2}\geq \frac{2(x+y)^2}{(2-(x+y))^2}$
$\Rightarrow A\geq \frac{2(2t)^2}{(2-2t)^2}=\frac{2t^{2}}{(1-t)^2}$
Do $0< t\leq \frac{1}{\sqrt[3]{2}}$ nên $\Rightarrow \sqrt{\frac{A}{2}}= \frac{t}{1-t}=\frac{1}{1-t}-1\geq \frac{1}{1-\frac{1}{\sqrt[3]{2}}}-1=\frac{1}{\sqrt[3]{2}-1}$
$\Rightarrow A\geq \frac{2}{(\sqrt[3]{2}-1)^2}$
Dấu $=$ xảy ra khi $x=y=\frac{1}{\sqrt[3]{2}}$
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