第87页 - 经纶学典新课时作业八年级数学上下册【江苏国标】

D

1

$-\frac{x²}{y}$

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

$ \begin{aligned}解:原式&=\frac{2(x-3)}{(x-2)²}·\frac{(x-2)(x+3)}{-(x-3)} \\ &=-\frac{2(x+3)}{x-2} \\ &=-\frac {2x+6}{x-2} \\ \end{aligned}$

$ \begin{aligned}解:原式&=\frac{(x²+y²)(x-y)(x+y)}{x(x-y)²}·\frac{x}{x²+y²} \\ &=\frac {x+y}{x-y} \\ \end{aligned}$

$由于x=4，y=-1，所以原式=\frac{3}{5}$

$ \begin{aligned}解:原式&=\frac{(a+b+b)(a+b-b)}{a(a+b)}·\frac{2b(a+b)}{(a+2b)²} \\ &=\frac{a(a+2b)}{a(a+b)}·\frac {2b(a+b)}{(a+2b)²} \\ &=\frac{2b}{a+2b} \\ \end{aligned}$

$由于a-2b=0，所以原式=\frac{1}{2}$

2

$解:根据题意得$

$ \begin{aligned} \frac{600}{(a-1)²}÷\frac{600}{a²-1}&=\frac{600}{(a-1)²}×\frac{(a-1)(a+1)}{600} \\ &=\frac {a+1}{a-1}. \\ \end{aligned}$

$ 答:“优选2号”水稻的单位面积产量是“优选1号”水稻的单位$

$面积产量的\frac {a+1}{a-1}倍.$