6. 解：

5. 解：(1)设A市投资“改水工程”年平均增长率是x，则

解之，得x＝0.4或x＝－2.4(不合题意，舍去)

所以，A市三年共投资“改水工程”2616万元.

4. 解法一：因为，所以．·················· 3分

即．所以，原方程的根为，．························· 6分

解法二：配方，得．··················································································· 2分

直接开平方，得．····················································································· 4分

所以，原方程的根为，．　 6分

3. 解：　　　 ………………1分

　　 　　　　　………………2分

　　　　　 ………………3分

∴x－1＝或x－1＝－ ………………4分

∴＝1+，＝1－　 ………………6分

2. x₁＝2　x₂＝　

1.

解：·································································································· 3分

或····································································································· 5分

，······································································································· 6分

1. ；2. -1；3. ；4. 10;5. ;6. ;7. 6或10或12;　 8.　 0；　 9.　 ，；　 10. 7，3；11. ，；　 12. ； 13.　 4；　 14.　 ；15. －4；16.　 4；　 17. 2；18. 　　19. 10%；　 20.　 +40－75=0 ； 21.、

1.A 2.D 3.A 4.B 5.C 6.C 7.B 8.D 9.B 10.C 11.A 12.A 13.A 14.A 15.D 16.D 17.D 18.D 19.B 20.A 21.C 22.A 23.B 24.B 25.B 26.D 27.C 28.B 29.A 30.B 31.C 32.A

27. (2008山东济宁)用配方法解方程：．

一元二次方程答案

26.(2008浙江温州)我们已经学习了一元二次方程的四种解法：因式分解法，开平方法，配方法和公式法．请从以下一元二次方程中任选一个，并选择你认为适当的方法解这个方程．

①；②；③；④．