\[\boxed{\text{561.}\text{\ }\text{ОК\ ГДЗ\ -\ домашка\ на\ 5}}\]
\[Пусть\ a = x;\ \ b = x + d;\ \ \]
\[тогда\ \ c = x + 2d;\]
\[a^{2} + ac + c^{2} =\]
\[= x^{2} + x(x + 2d) + (x + 2d)^{2} =\]
\[= x^{2} + x^{2} + 2xd + x^{2} + 4xd + 4d^{2} =\]
\[= 3x^{2} + 6xd + 4d^{2} =\]
\[= \left( 3x^{2} + 3dx + d^{2} \right) + \left( 3dx + 3d^{2} \right);\]
\[членами\ арифметической\ \]
\[прогрессии.\]