ta có \(\frac{11b^3-a^3}{ab+4b^2}+\frac{11c^3-b^3}{bc+4c^2}+\frac{11a^3-c^3}{ca+4a^2}=\frac{11-\left(\frac{a}{b}\right)^3}{\frac{a}{b}+4}\cdot b+\frac{11-\left(\frac{b}{c}\right)^3}{\frac{b}{c}+4}\cdot c+\frac{11-\left(\frac{c}{a}\right)^3}{\frac{c}{a}+4}\cdot a\)
khi a=b=c=1 ta thấy đẳng thức xảy ra
xét \(f\left(x\right)=\frac{11-x^3}{x+4}\)ta có \(\frac{11-x^3}{x+4}\le-x+3\Leftrightarrow\left(x-1\right)^2\left(x+1\right)\ge0\forall x>0\)
thay x bởi a/b ta được \(\frac{11-\left(\frac{a}{b}\right)^3}{\frac{a}{b}+4}\le-\frac{a}{b}+3\Leftrightarrow\frac{11b^3-a^3}{ab+4b^2}\le-a+3b\)
tương tự \(\hept{\begin{cases}\frac{11c^3-b^3}{bc+4c^2}\le-b+3c\\\frac{11ba^3-c^3}{ac+4a^2}\le-c+3a\end{cases}}\)
cộng các bđt cùng chiều ta được
\(\frac{11b^3-a^3}{ab+4b^2}+\frac{11c^3-b^3}{bc+4c^2}+\frac{11a^3-c^3}{ac+4a^2}\le2\left(a+b+c\right)=6\)
\(\frac{11b^3-a^3}{ab+4b^2}\le3b-a\)
