第72页 - 江苏版启东中学作业本八年级数学（上下册）

C

D

A

$x$

$\frac{1}{x-1}$

$\frac{k+1}{k-1}$

$ \begin{aligned}解：原式&=\frac{x+4-1}{x^2+3x} \\ &=\frac{x+3}{x(x+3)} \\ &=\frac{1}{x} \\ \end{aligned}$

$ \begin{aligned}解：原式&=\frac{x-3+1-1+x}{x-3} \\ &=\frac{2x-3}{x-3} \\ \end{aligned}$

$ \begin{aligned}解：原式&= \frac{x+1}{(x+1)(x-1)}+\frac{x^2-3x}{(x+1)(x-1)} \\ &=\frac{x^2-2x+1}{(x+1)(x-1)} \\ &=\frac{(x-1)^2}{(x+1)(x-1)} \\ &=\frac{x-1}{x+1} \\ \end{aligned}$

$ \begin{aligned} 解：原式&=\frac{2x^2}{x+y}-(x-y) \\ &=\frac{2x^2}{x+y}-\frac{(x+y)(x-y)}{x+y} \\ &=\frac{2x^2-x^2+y^2}{x+y} \\ &=\frac{x^2+y^2}{x+y} \\ \end{aligned}$