Решение задания 20

1. $$\frac{5b}{28} + \frac{9b}{28} = \frac{5b+9b}{28} = \frac{14b}{28} = \frac{b}{2}$$ Ответ: $$\frac{b}{2}$$
2. $$\frac{9m}{7n} - \frac{2m}{7n} = \frac{9m-2m}{7n} = \frac{7m}{7n} = \frac{m}{n}$$ Ответ: $$\frac{m}{n}$$
3. $$\frac{5x-3y}{8z} + \frac{3x-13y}{8z} = \frac{5x-3y+3x-13y}{8z} = \frac{8x-16y}{8z} = \frac{8(x-2y)}{8z} = \frac{x-2y}{z}$$ Ответ: $$\frac{x-2y}{z}$$
4. $$\frac{4c-3d}{cd} - \frac{c-3d}{cd} = \frac{4c-3d-(c-3d)}{cd} = \frac{4c-3d-c+3d}{cd} = \frac{3c}{cd} = \frac{3}{d}$$ Ответ: $$\frac{3}{d}$$