\[\boxed{\mathbf{625\ (625)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\frac{a}{b} = \frac{c}{d}\]
\[1)\ \frac{a - b}{b} = \frac{c - d}{d}\]
\[\frac{a}{b} - \frac{b}{b} = \frac{c}{d} - \frac{d}{d}\]
\[\frac{a}{b} = \frac{c}{d}\]
\[\Longrightarrow ч.т.д.\]
\[2)\ \frac{a}{a + b} = \frac{c}{c + d}\]
\[a(c + d) = c(a + b)\]
\[ad = bc\]
\[\frac{a}{b} = \frac{c}{d}\]
\[\Longrightarrow ч.т.д.\]