21.(2008　 河南)先化简，再求值：－÷,其中a＝1－

解：原式＝－×

　　　　 ==

当=1时，原式==

20. (2008年陕西省)先化简，再求值：，其中，．

解：原式

当，时，原式　　　 (6分)

19. (2008桂林市)

有一道题：“先化简再求值：，其中”，小明做题时把“”错抄成了“”，但他的计算结果也是正确，请你通过计算解释这是怎么回事？

解：

＝

＝

＝

∵当或时，的值均为2008，

∴小明虽然把值抄错，但结果也是正确的.

18.(08湖南常德市)化简：

解：原式＝…………………………2分

　　＝＝＝………………5分

17. (2008黑龙江黑河)先化简：，再任选一个你喜欢的数代入求值．

解：

取和2以外的任何数

16. (2008山东德州)先化简，再求值：

÷，其中，．　

解：原式＝ ＝＝．

当，时，原式＝

15. (2008年乐山市)已知，求代数式的值

解：化简得原式=，代入得=

14.(2008年成都市)化简：

解：原式=2x-1+　　　　　　　　　　　 ……4分

　　　　　　 =2x-1+x+2

　　　　　　 =3x+1　　　　　　　　　　　　　　　　　　　　　　 ……2分

13. (2008年四川巴中市)在解题目：“当时，求代数式的值”时，聪聪认为只要任取一个使原式有意义的值代入都有相同结果．你认为他说的有理吗？请说明理由．

解：聪聪说的有理．······································································································ 1分

················································································ 3分

··················································································································· 4分

······························································································································· 5分

只要使原式有意义，无论取何值，原式的值都相同，为常数1．····························· 6分

12. .(2008年辽宁省十二市)先化简，再求值：，其中．

解法一：原式·································································· 2分

······················································································································· 6分

当时，原式···················································································· 8分

解法二：原式··········································· 2分

······················································································································· 6分

当时，原式···················································································· 8分