第87页 - 亮点给力提优课时作业本八年级数学江苏版

m

$=\frac {a-1}{a+2} · \frac {(a+2)(a-2)}{(a-1)²}$

$=\frac {a-2}{a-1}$

$=\frac {x²-2x+1}{x} · \frac {x}{(x+1)(x-1)}$

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

解：由题意，得$(xy-x²)÷\frac {x²-2xy+y^2}{xy}= \frac {x^2}{x-y}×D$

∴$x(y-x) · \frac {xy}{(x-y)²}=\frac {x^2}{x-y}×D$，即$D=\frac {x^2y}{y-x}÷ \frac {x^2}{x-y}=\frac {x^2y}{y-x} · \frac {x-y}{x^2}=-y$

A

$\frac {7}{4} $

解：设花种数量为$m$

由题意得甲地的播撒密度是$ \frac {m}{a²-b²}$，

乙地的播撒密度是$\frac {m}{\frac {1}{4}(a+b)²}$

∴甲、$ $乙两块土地的播撒密度比为$ \frac {m}{a^2-b²} ÷\frac {m}{\frac {1}{4}(a+b)²} =\frac {a+b}{4(a-b)} $

当$ a=\frac {5}{3}\ \mathrm {b} $时，原式$= 1$

∴当$a=\frac {5}{3}b$时，甲、乙两块土地的播撒密度一样大