Đáp án: `+` Giải thích các bước giải:

 `7. \frac{1}{xy - x^2} - \frac{1}{y^2 - xy}`

`= \frac{1}{x(y - x)} - \frac{1}{y(y - x)}`

`= \frac{y}{xy(y - x)} - \frac{x}{xy(y - x)}`

`= \frac{y - x}{xy(y - x)}`

`= \frac{1}{xy}`

`8. \frac{x + 1}{x + 4} - \frac{x^2 - 4}{x^2 - 16}`

`= \frac{x + 1}{x + 4} - \frac{x^2 - 4}{(x - 4)(x + 4)}`

`= \frac{(x + 1)(x - 4)}{(x + 4)(x - 4)} - \frac{x^2 - 4}{(x + 4)(x - 4)}`

`= \frac{x^2 - 4x + x - 4 - x^2 + 4}{(x + 4)(x - 4)}`

`= \frac{-3x}{x^2 - 16}`

`9.\frac{1 - 3x}{2x} + \frac{3x - 2}{2x - 1} + \frac{3x - 2}{2x - 4x^2}`

`= \frac{1 - 3x}{2x} + \frac{3x - 2}{2x - 1} + \frac{3x - 2}{2x(1 - 2x)}`

`= \frac{1 - 3x}{2x} + \frac{3x - 2}{2x - 1} - \frac{3x - 2}{2x(2x - 1)}`

`= \frac{(1 - 3x)(2x - 1) + 2x(3x - 2) - (3x - 2)}{2x(2x - 1)}`

`= \frac{2x - 1 - 6x^2 + 3x + 6x^2 - 4x - 3x + 2}{2x(2x - 1)}`

`= \frac{-2x + 1}{2x(2x - 1)}`

`= \frac{-(2x - 1)}{2x(2x - 1)}`

`= \frac{-1}{2x}`