\[10(x - 2) \leq x^{2} + 5\]
\[10x - 20 \leq x^{2} + 5\]
\[x^{2} - 10x + 25 \geq 0\]
\[(x - 5)^{2} \geq 0\]
\[Что\ и\ требовалось\ доказать.\]