\[x(x + 10) < (x + 5)²\]
\[x^{2} + 10x < x^{2} + 10x + 25\]
\[x^{2} + 10x - x^{2} - 10x - 25 < 0\]
\[- 25 < 0 \Longrightarrow верно.\]