19. 解：原式 ．

　　 　　∴當(dāng)時(shí)，原式．

18. 解：原式=

當(dāng)x=-1,y=時(shí)

原式=

17.

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

(2)當(dāng)a=11時(shí)，s=1581×11+1609=19000(米)

14.(1) 解：原式＝1+－

　�。�1+－

　　＝1

(2) 解：原式＝

　�。�

　　＝

取x＝10，則原式＝10

15　 5

13.、解：

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

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

當(dāng)時(shí)，分式無意義．

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

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

=×–×

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

=–

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

當(dāng)x=1時(shí)，

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

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

說明：以上步驟可合理省略

11. 解：原式=

　　　　　　　　 =………………2分

　　　　　　　　 =x－4 ………………3分

　　　　　　　　 當(dāng)時(shí)， 原式=………………4分

10.

解:原式=…=　　　

由得，

代入原式=2　

9. 解:原式=