zaa.[math]\sqrt{x+5}-y^3=\sqrt{y+5}-x^3[/math][math]\Leftrightarrow (\sqrt{x+5}-\sqrt{y+5})+(x^3-y^3)=0[/math][math]\Leftrightarrow \dfrac{x-y}{\sqrt{x+5}+\sqrt{y+5}}+(x-y)(x^2+xy+y^2)=0[/math][math]\Leftrightarrow (x-y)(\dfrac{1}{\sqrt{x+5}+\sqrt{y+5}}+x^2+xy+y^2)=0[/math][math]\Leftrightarrow \left[\begin{array}{l} x=y\\\dfrac{1}{\sqrt{x+5}+\sqrt{y+5}}+x^2+xy+y^2=0 (*)\end{array}\right.[/math](*) VT >0 nên vô nghiệm
suy ra x=y
Thế vào P
[imath]P=-3x^2+12x+2011=-3(x-2)^2+2023 \leq 2023[/imath]
Dấu bằng xảy ra khi x=y=2