\[(a + b)\left( ab - a^{2} + ab - b^{2} \right) \leq 0\]
\[- (a + b)(a - b)^{2} \leq 0\ \ \]
\[так\ как\ a + b \geq 0;\ \ \ \]
\[(a - b)^{2} \geq 0;то\]
\[- (a + b)(a - b)^{2} \leq 0 \Longrightarrow верно.\]