Ta có:
\(4\le\left(\sqrt{a}+1\right)\left(\sqrt{b}+1\right)=\sqrt{ab}+\sqrt{a}+\sqrt{b}+1\le\dfrac{a+b}{2}+\dfrac{a+1}{2}+\dfrac{b+1}{2}+1\)
\(=a+b+2\)
\(\Leftrightarrow a+b\ge2\)
\(\dfrac{a^2}{b}+\dfrac{b^2}{a}\ge\dfrac{\left(a+b\right)^2}{a+b}=a+b\ge2\)
Dấu \(=\) xảy ra khi \(a=b=1\).