\[x^{2} - 8x + 17 > 0\]
\[\left( x^{2} - 8x + 16 \right) + 1 > 0\]
\[(x - 4)^{2} + 1 > 0\]
\[(x - 4)^{2} \geq 0\ при\ любом\ x,\ \ \]
\[1 > 0 \Longrightarrow x^{2} - 8x + 17 > 0\ \Longrightarrow\]
\[\Longrightarrow ч.т.д.\]