4.8. а) \(\frac{x}{7y}-\frac1y\); б) \(\frac{a}{12b}+\frac{3a}{4b}\); в) \(\frac7a+\frac{b}{5a}\); г) \(\frac{5y}{8x}-\frac{7y}{24x}\).

1. а) \(\frac{x}{7y}-\frac1y=\frac{x}{7y}-\frac7{7y}=\frac{x-7}{7y}\).
2. б) \(\frac{a}{12b}+\frac{3a}{4b}=\frac{a}{12b}+\frac{9a}{12b}=\frac{10a}{12b}=\frac{5a}{6b}\).
3. в) \(\frac7a+\frac{b}{5a}=\frac{35}{5a}+\frac{b}{5a}=\frac{b+35}{5a}\).
4. г) \(\frac{5y}{8x}-\frac{7y}{24x}=\frac{15y}{24x}-\frac{7y}{24x}=\frac{y}{3x}\).

Знаменатели должны быть ненулевыми.