\[\boxed{\text{818.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[n^{2} + (n + 2)^{2} + (n + 9)^{2} = n^{2} +\]
\[+ n^{2} + 4n + 4 + n^{2} +\]
\[+ 18n + 81 =\]
\[= 3n^{2} + 22n + 85\]
\[(n - 1)^{2} + (n + 5)^{2} +\]
\[+ (n + 7)^{2} + 10 =\]
\[= n^{2} - 2n + 1 + n^{2} + 10n +\]
\[+ 25 + n^{2} + 14n + 49 +\]
\[+ 10 = 3n^{2} + 22n + 85\]
\[Значит:\]
\[n^{2} + (n + 2)^{2} + (n + 9)^{2} =\]
\[= (n - 1)^{2} + (n + 5)^{2} +\]
\[+ (n + 7)^{2} + 10\]
\[Подставим\ n = 3:\]
\[3^{2} + (3 + 2)^{2} + (3 + 9) = 9 +\]
\[+ 25 + 144 = 178\]
\[(3 - 1)^{2} + (3 + 5)^{2} + (3 + 7)^{2} +\]
\[+ 10 = 4 + 64 + 100 +\]
\[+ 10 = 178\]
\[Данное\ равенство\ верно\ при\ \]
\[любом\ n,\ так\ как\ его\ левая\ \]
\[сторона\ \]
\[равна\ правой\ стороне.\]