Решение задания 77

a)

$$\frac{15a-b}{12a} - \frac{a-4b}{9a} = \frac{3(15a-b) - 4(a-4b)}{36a} = \frac{45a - 3b - 4a + 16b}{36a} = \frac{41a+13b}{36a}$$

б)

$$\frac{7x+4}{8y} - \frac{3x-1}{6y} = \frac{3(7x+4) - 4(3x-1)}{24y} = \frac{21x + 12 - 12x + 4}{24y} = \frac{9x + 16}{24y}$$