Làm tiếp ạ
\(\Rightarrow P\ge\frac{289}{16}\)
Dấu"="Xảy ra \(\Leftrightarrow x=y=\frac{1}{2}\)
Vậy MIN P=\(\frac{289}{16}\)\(\Leftrightarrow x=y=\frac{1}{2}\)
Em chả có cách gì ngoài cô si mù mịt :v
\(\left(x^2+\frac{1}{y^2}\right)\left(y^2+\frac{1}{x^2}\right)\)
\(=\left(x^2+\frac{1}{16y^2}+\frac{1}{16y^2}+.....+\frac{1}{16y^2}\right)\left(y^2+\frac{1}{16x^2}+\frac{1}{16x^2}+.....+\frac{1}{16x^2}\right)\)
\(\ge17\sqrt[17]{\frac{x^2}{16^{16}\cdot y^{32}}}\cdot17\sqrt[17]{\frac{y^2}{16^{16}\cdot x^{32}}}\)
\(=17^2\sqrt[17]{\frac{x^2y^2}{16^{32}\cdot x^{32}\cdot y^{32}}}\)
\(=17^2\sqrt[17]{\frac{1}{16^{32}\cdot\left(xy\right)^{30}}}\)
\(\ge17^2\sqrt[17]{\frac{1}{16^{32}\left(\frac{x+y}{2}\right)^{60}}}=\frac{289}{16}\)
Dấu "=" xảy ra tại x=y=1/2
\(P=\left(x^2+\frac{1}{y^2}\right)\left(y^2+\frac{1}{x^2}\right)\)
\(=x^2y^2+\frac{1}{x^2y^2}+2\)
\(=x^2y^2+\frac{1}{256x^2y^2}+\frac{255}{256x^2y^2}+2\)
Áp dụng BĐT AM-GM ta có:
\(x^2y^2+\frac{1}{256x^2y^2}\ge2\sqrt{x^2y^2.\frac{1}{256x^2y^2}}=\frac{1}{8}\)
\(xy\le\frac{\left(x+y\right)^2}{4}\Rightarrow x^2y^2\le\frac{\left(x+y\right)^4}{16}=\frac{1}{16}\)
\(\Rightarrow P\ge\frac{1}{8}+\frac{255}{256.\frac{1}{16}}+2\)