模考後檢討的錯題
When the positive integer n is divided by 25, the remainder is 13
what is the value of n?
1. n<100
2. When n is divided by 20, the remainder is 3
是C
請問選B的思維錯在哪呢?
我由題目的 integer n is divided by 25, the remainder is 13和條件二的When n is divided by 20, the remainder is 3 我設兩個方程式 n=25a+13, n=20b+3
再讓這兩個式子相等得到 25a+13=20b+3 所以選B
請問選B的思維錯在哪呢?作者: vansonkated 时间: 2011-6-19 21:21
我是这样看的，你这样思路没错，得到等式25a+13=20b+3也没错
化简，得到b=(5a+2)/4；由于都是整数，得到两组解；b=3 a=2; b=8 a=6
这样的话B就不能确定了
但是如果有n<100这个条件的话，后面得就可以舍去 得到唯一解
hope it can be of help.作者: teddyabc 时间: 2011-6-23 06:26
