\(f\left(x\right)\le\sqrt{\left(2^2+1^2\right)\left(x-4+8-x\right)}=2\sqrt{5}\)
Dấu "=" xảy ra khi \(\frac{x-4}{4}=8-x\Rightarrow x=\frac{36}{5}\)
\(f\left(x\right)=\sqrt{x-4}+\sqrt{8-x}+\sqrt{x-4}\)
\(f\left(x\right)\ge\sqrt{x-4+8-x}+\sqrt{x-4}=2+\sqrt{x-4}\ge2\)
Dấu "=" xảy ra khi \(x=4\)