Question

（2007•西城區(qū)一模）下列等式中不成立的是（　�。�

• A．

  xy

  x2-xy

  =

  y

  x-y

• B．

  y

  x

  -

  x

  y

  =

  y2-x2

  xy

• C．

  x2-2xy+y2

  x-y

  =x-y
• D．

  x2-y2

  x-y

  =x-y

Answer 1

分析：先將分式的分子、分母分解因式，然后根據(jù)分式的基本性質(zhì)進(jìn)行化簡就可以求出其結(jié)論．

解答：解：A：原式=

xy

x2-xy

=

xy

x(x-y)

=

y

x-y

，故本答案正確．
B：原式=

y

x

-

x

y

=

y2

xy

-

x2

xy

= 

y2-x2

xy

，故本答案正確．
C：原式=

x2-2xy+y2

x-y

=

(x-y)2

x-y

=x-y，本答案正確．
D：原式=

x2-y2

x-y

=

(x-y)(x+y)

x-y

=x+y，本答案錯(cuò)誤．
故選D．

點(diǎn)評(píng)：本題考查了分式的約分，涉及了對分式基本性質(zhì)的運(yùn)用，因式分解的運(yùn)用．