\[\boxed{\text{53\ (53).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[\textbf{а)}\ \frac{x}{3} + \frac{y}{3} = \frac{x + y}{3}\]
\[\textbf{б)}\ \frac{5b^{2}}{a} - \frac{13b^{2}}{a} = \frac{5b^{2} - 13b^{2}}{a} =\]
\[= \frac{- 8b^{2}}{a}\]
\[\textbf{в)}\ \frac{x + y}{9} - \frac{x}{9} = \frac{x + y - x}{9} = \frac{y}{9}\]
\[\textbf{г)}\ \frac{2c - x}{b} + \frac{x}{b} = \frac{2c - x + x}{b} = \frac{2c}{b}\]