\[\sqrt{25 - 10a + a^{2}} =\]
\[= \sqrt{\left( a^{2} - 10a + 25 \right)} =\]
\[= \sqrt{(a - 5)^{2}} = |a - 5|\]
\[a = 3,7:\]
\[|a - 5| = |3,7 - 5| = | - 1,3| = 1,3.\]
\[Ответ:1,3.\]