\(BDT\Leftrightarrow\sum\left[\dfrac{\left(a+b\right)^2}{c^2+ab}-2\right]\ge0\)\(\Leftrightarrow\sum\dfrac{a^2+b^2-2c^2}{c^2+ab}\ge0\)(*)
\(\Leftrightarrow\sum\left(\dfrac{a^2-c^2}{c^2+ab}+\dfrac{b^2-c^2}{c^2+ab}\right)\ge0\)
\(\Leftrightarrow\sum\left(c^2-a^2\right)\left(\dfrac{1}{a^2+bc}-\dfrac{1}{c^2+ab}\right)\ge0\)
\(\Leftrightarrow\sum\left(c-a\right)^2.\dfrac{\left(c+a\right)\left(c+a-b\right)}{\left(a^2+bc\right)\left(c^2+ab\right)}\ge0\)
\(\dfrac{\left(a+b\right)^2}{c^2+ab}+\dfrac{\left(b+c\right)^2}{a^2+bc}+\dfrac{\left(c+a\right)^2}{b^2+ca}\ge\dfrac{\left(a+b+b+c+c+a\right)^2}{a^2+b^2+c^2+ab+bc+ca}\)\(=\dfrac{4\left(a+b+c\right)^2}{a^2+b^2+c^2+ab+bc+ca}\) (theo AM-GM với a ; b>0)
\(=\dfrac{4\left(a^2+b^2+c^2+2ab+2bc+2ca\right)}{a^2+b^2+c^2+ab+bc+ca}=\dfrac{4.3.\left(a^2+b^2+c^2\right)}{2.\left(a^2+b^2+c^2\right)}\)(do \(a^2+b^2+c^2\ge ab+bc+ca\))
\(=4.1,5\) = 6 ( do a;b;c>0)
