Áp dụng bđt bunhiacopski có:
\(\left(a^4+1\right)\left(1+4^2\right)\ge\left(a^2+4\right)^2\)
=> \(\sqrt{a^4+1}\ge\sqrt{\frac{\left(a^2+4\right)^2}{1+4^2}}=\frac{a^2+4}{\sqrt{17}}\)(1)
Tương tự cx có: \(\sqrt{b^4+1}\ge\frac{b^2+4}{\sqrt{17}}\) (2)
Từ (1),(2) => \(F\ge\frac{a^2+b^2+8}{\sqrt{17}}\)
Có (a+2)(b+2)=\(\frac{25}{4}\)
=> \(ab+2a+2b+4=\frac{25}{4}\) <=> \(ab+2a+2b=\frac{9}{4}\)
Áp dụng cosi có:
\(ab\le\frac{a^2+b^2}{2}\)
\(2a\le2\left(a^2+\frac{1}{4}\right)\)
\(2b\le2\left(b^2+\frac{1}{4}\right)\)
=> \(\frac{a^2+b^2}{2}+2a^2+\frac{1}{2}+2b^2+\frac{1}{2}\ge ab+2a+2b=\frac{9}{4}\)
<=> \(\frac{a^2+b^2+4a^2+4b^2}{2}\ge\frac{9}{4}-\frac{1}{2}-\frac{1}{2}=\frac{5}{4}\)
<=> \(\frac{5\left(a^2+b^2\right)}{2}\ge\frac{5}{4}\)
<=> \(a^2+b^2\ge\frac{1}{2}\)
Thay \(a^2+b^2\ge\frac{1}{2}\) vào F có:
\(F\ge\frac{\frac{1}{2}+8}{\sqrt{17}}\)
<=> F \(\ge\frac{\sqrt{17}}{2}\)
Dấu "=" xảy ra <=>\(a=b=\frac{1}{2}\)
