Dk x<=3
\(!x!>\sqrt{3-x}\)
\(\sqrt{3-x}=t\ge0\Rightarrow t^2=3-x\Rightarrow x=3-t^2\)
\(\Leftrightarrow A=t+3-t^2=3+t-t^2=3+\frac{1}{4}-\left(\frac{1}{4}-2.\frac{1}{2}t+t^2\right)\)
\(A=\frac{13}{4}-\left(t-\frac{1}{2}\right)^2\ge\frac{13}{4}\) đẳng thức khi t=1/2\(\Rightarrow x=3-\frac{1}{4}=\frac{11}{4}\)
Kết luận Amax \(=\frac{13}{4}\) khi \(x=\frac{11}{4}\)