\[x^{2} - 6x + 13 = x^{2} - 6x + 9 + 4 =\]
\[= (x - 3)^{2} + 4 > 0\ при\ любом\ x,\]
\[так\ как\ (x - 3)^{2} \geq 0;\ \ 4 > 0.\]