75. 解：去分母，得

．·············································································· (3分)

整理，得．····················································································· (2分)

，．····························································································· (4分)

经检验，是增根，是原方程的根．··················································· (1分)

所以，原方程的根是．

74. 解：原式＝　　 ……………………………2分

＝　　　　 …………………………………………3分

＝．　　　 ……………………………………………………………4分

当，时，

原式＝．　

73. 解：方程两边都乘以，得：

　　　　　　　　　　　　 解得：

　　　　　　　　　　　　 经检验：是原方程的根；

72. ．解：设原计划每小时抢修的路线长为，根据题意，得

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

解之得

························································································································ 7分

检验：是原方程的解，且符合题的实际意义．

答：原计划每小时抢修的路线长为400m．···································································· 8分

71. 解：原式················································································ 2分

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

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

选取除0与1以外的任何值，求代数式的值　　 6分

70. (1)设乙工程队单独完成建校工程需天，则甲工程队单独完成建校工程需，依题意得：

．·········································································································· 3分

解得，经检验是原方程的解，，

所以甲需180天，乙需120天；····················································································· 4分

(2)甲工程队需总费用为(万元)，······························· 5分

设乙工程队施工时平均每天的费用为，则，···················· 7分

解得，

所以乙工程队施工时平均每天的费用最多为万元．　　 8分

69. (1)2000

　(2)设该公司原计划安排x名工人生产帐篷，则由题意得：

　　∴

∴解这个方程，得：x＝750

经检验：x＝750是所列方程的解

68. 解：原式错误！不能通过编辑域代码创建对象。　错误！不能通过编辑域代码创建对象。　错误！不能通过编辑域代码创建对象。　　　　　　　　　　

当x=－4时，原式=错误！不能通过编辑域代码创建对象。　　　　　

67. 解：原式=

取a=2，原式=2008.(取a=3，原式=1004)

66.