\[a² - 10a + 26 > 0\]
\[a^{2} - 10a + 25 + 1 > 0\]
\[(a - 5)^{2} + 1 > 0\]
\[(a - 5)^{2} \geq 0;\ \ 1 > 0 \Longrightarrow верно.\]