$\frac{7}{x-y} + \frac{5}{y-x} =$ $\frac{x+1}{x-1} + \frac{1}{1-x} =$ $\frac{x^2}{x-b} + \frac{y^2}{y-x} =$ $\frac{3}{a-b} + \frac{2}{a} =$ $\frac{x}{x-3} + \frac{2}{x} =$ $\frac{3}{a-4} - \frac{2}{a+4} =$ $\frac{x-2}{x} - \frac{x}{x+2} =$ $\frac{x+1}{1-x} + 1 =$ $\frac{a}{a+1} - 1 =$ $\frac{1}{x+2} + 2 =$