临考急问数学一题

Luluisland · 发表于 2007-5-7 19:32:00

On his trip from Alba to Bento, Julio drove the first x miles at an average rate of 50 miles per hour and the remaining distant at an average rate of 60 miles per hour, how long did it take Julio to drive the x miles?

1) on this trip, Julio drove for a total of 10 hours and drove a total of 530 miles
2) on this trip, it took Julio 4 more hour to drive the first x miles than to drive the remaining distance

The answer is A.为什么条件2不可以那？请多多指教啊！

darkage · 发表于 2007-5-7 19:58:00

总的距离不知道,所以2不可以!

crowkiller · 发表于 2007-5-7 20:24:00

假设总路程为 S , first x miles 跑了a 小时，remaining distant 跑了b小时, 原题要求a 的值
则有 s = x + 60*b
x = 50 * a

1) ==> S =530
       a+b = 10       ===> x = 350 ; a= 7
       s = x + 60*b
       x = 50 * a

2) ==> a = b+4
s = x + 60*b ===> S 不知道，无法求得a 的值。
x = 50 * a

===》答案为A

像这样的应用题，建议象这样列出来方程式来看能不能求出解，这样比较不会漏掉或者多加上条件。

加油！

[此贴子已经被作者于2007-5-7 20:27:49编辑过]

Luluisland · 发表于 2007-5-7 20:59:00

多谢LS二位！

临考急问数学一题

临考急问数学一题

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