摘要：21．题5分.第 (1) 解:原式 ································································· 4分 ······························································································ 5分 (2) 解:方程两边同乘.得 ································································· 1分 ································································· 3分 解这个方程.得 x=2 ············································································· 4分 检验:当x=2时.=0.所以x=2是增根.原方程无解．················ 5分
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，求分式
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，求分式
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（本题7分）某市为了鼓励居民节约用水，采用分段计费的方法按月计算每户家庭的水费，月用水量不超过20立方米时，按2元/立方米计费；月用水量超过20立方米时，其中的20立方米仍按2元/立方米收费，超过部分按2.6元/立方米计费．设每户家庭用水量为x立方米时，应交水费y元．
（1）当0≤x≤20时，y与x的函数关系式为 ；
当x＞20时，y与x的函数关系式为 。
（2）小明家第二季度交纳水费的情况如下：
| 月份 | 四月份 | 五月份 | 六月份 |
| 交费金额 | 30元 | 34元 | 47.8元 |