3. (2008年云南省双柏县)先化简，再求值：
，其中．
解：解：原式
将代入上式得
原式
1、(2008山东烟台)已知，求的值.
2(2008年沈阳市)先化简，再求值：
，其中，．
解：解：原式······················································· 4分
·························································································································· 6分
当，时，
原式···································································································· 8分
20.
19. (2008年镇江)如果，则 ； ．
答案：1，1
18. (2008遵义)计算：()2÷＝________.
答案：4a
17. (08宁德)计算：＝________________.
答案：-2m
16. (2008海南省)计算： .
答案：
15. (2008佛山)计算： .
14. (2008 山东 聊城)计算： ．
13.(2008山西省高中)计算： 。
，其中
．
，求
的值.
，其中
，
．
······················································· 4分
·························································································································· 6分
···································································································· 8分
，则
；
．
)2÷
＝________.
＝________________.
. 
. 
．
。