Решение:

1. $$\frac{1}{2} + \frac{x}{3} = \frac{3}{6} + \frac{2x}{6} = \frac{3+2x}{6}$$
2. $$\frac{2}{5} + \frac{a}{2} = \frac{4}{10} + \frac{5a}{10} = \frac{4+5a}{10}$$
3. $$\frac{y}{7} - \frac{x}{3} = \frac{3y}{21} - \frac{7x}{21} = \frac{3y-7x}{21}$$
4. $$\frac{2}{a} + \frac{1}{3} = \frac{6}{3a} + \frac{a}{3a} = \frac{6+a}{3a}$$