Do \(1\le x\le2\Rightarrow\left(x-1\right)\left(x-2\right)\le0\)
\(\Leftrightarrow x^2+2\le3x\)
Tương tự \(y^2+2\le3y\)
Do đó:
\(P=\frac{x+2y}{x^2+2+3y+3}+\frac{2x+y}{y^2+2+3x+3}+\frac{1}{4\left(x+y-1\right)}\ge\frac{x+2y}{3x+3y+3}+\frac{2x+y}{3x+3y+3}+\frac{1}{4\left(x+y-1\right)}\)
\(P\ge\frac{3x+3y}{3x+3y+3}+\frac{1}{4\left(x+y-1\right)}=\frac{x+y}{x+y+1}+\frac{1}{4\left(x+y-1\right)}\)
Đặt \(x+y=t\Rightarrow2\le t\le4\)
\(\Rightarrow P\ge\frac{t}{t+1}+\frac{1}{4t-4}=\frac{t}{t+1}+\frac{1}{4t-4}-\frac{7}{8}+\frac{7}{8}\)
\(P\ge\frac{\left(t-3\right)^2}{8\left(t^2-1\right)}+\frac{7}{8}\ge\frac{7}{8}\)
\(P_{min}=\frac{7}{8}\) khi \(t=3\) hay \(\left(x;y\right)=\left(1;2\right);\left(2;1\right)\)
