18. 解：原式=

当x=-1,y=时

原式=

17.

16..解：(1)s=700(a－1)+(881a+2309)=1581a+1609

(2)当a=11时，s=1581×11+1609=19000(米)

14.(1) 解：原式＝1+－

　　＝1+－

　　＝1

(2) 解：原式＝

　　＝

　　＝

取x＝10，则原式＝10

15　 5

13.、解：

　 　　　=············································································ 3分

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

当时，分式无意义．

原式的值为2．······························································································ 6分

12. ．原式=[–]×······················································ 3分

=×–×

=–·································································································· 4分

=–

=··········································································································· 5分

当x=1时，

原式=····································································································· 6分

= 1················································································································· 7分

说明：以上步骤可合理省略

11. 解：原式=

　　　　　　　　 =………………2分

　　　　　　　　 =x－4 ………………3分

　　　　　　　　 当时， 原式=………………4分

10.

解:原式=…=　　　

由得，

代入原式=2　

9. 解:原式=　　　

8. 解：

······································································································ 2分

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

．···················································································································· 4分

当时，原式．······································································ 6分