AP DUNG \(a+b\le\sqrt{2\left(a^2+b^2\right)}\) (bạn tự chứng minh nhé)
ta co P= \(\sqrt{x-5}+\sqrt{13-x}\le\sqrt{2\left(x-5+13-x\right)}=\sqrt{36}=6\)
dau "=" xay ra khi va chi khi \(\sqrt{x-5}=\sqrt{13-x}\Leftrightarrow x=9\)
max P = 6 \(\Leftrightarrow x=9\)