Решение:

1. a) $$\frac{5x-7}{6x} - \frac{x-3}{6x} + \frac{2x-8}{6x} = \frac{5x-7-(x-3)+2x-8}{6x} = \frac{5x-7-x+3+2x-8}{6x} = \frac{6x-12}{6x} = \frac{6(x-2)}{6x} = \frac{x-2}{x}$$
2. б) $$\frac{8y-5}{7y} - \frac{2y-1}{7y} - \frac{10-y}{7y} = \frac{8y-5-(2y-1)-(10-y)}{7y} = \frac{8y-5-2y+1-10+y}{7y} = \frac{7y-14}{7y} = \frac{7(y-2)}{7y} = \frac{y-2}{y}$$
3. в) $$\frac{x-5}{x^2-49} + \frac{12}{x^2-49} = \frac{x-5+12}{x^2-49} = \frac{x+7}{x^2-49} = \frac{x+7}{(x-7)(x+7)} = \frac{1}{x-7}$$