\[\boxed{\text{212\ (212).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[Выражение\ не\ имеет\ смысла,\ \]
\[когда\ знаменатель\ равен\ 0.\]
\[\textbf{а)}\ \frac{5}{2x - 4}\text{\ \ }\]
\[2x - 4 = 0\]
\[2x = 4\]
\[\Longrightarrow при\ x = 2.\]
\[\textbf{б)}\ \frac{3}{4y + 2}\text{\ \ }\]
\[4y + 2 = 0\]
\[4y = - 2\]
\[y = - 2\ :4\]
\[y = - \frac{2}{4} = - \frac{1}{2}\]
\[\Longrightarrow при\ y = - 0,5.\]
\[\textbf{в)}\ \frac{a}{a - b}\text{\ \ }\]
\[a - b = 0\]
\[\Longrightarrow при\ a = b.\]
\[\textbf{г)}\ \frac{b}{a + b}\ \]
\[a + b = 0\]
\[\Longrightarrow при\ a = - b.\]