一道GMATPrep上的题

camilla2006

When a certain tree was first planted, it was 4 feet tall, and the height of the tree increased by a constant amount each year for the next 6 years. At the end of the 6th year, the tree was 1/5 taller than it was at the end of the 4th year. By how many feet did the height of the tree increase each year?

answer: 2/3

what does it meant when it says "the tree was 1/5 taller than it was at the end of the 4th "?

请教思路。谢谢。

kevin_chee20

以下是引用camilla2006在2006-6-28 16:07:00的发言：

When a certain tree was first planted, it was 4 feet tall, and the height of the tree increased by a constant amount each year for the next 6 years. At the end of the 6th year, the tree was 1/5 taller than it was at the end of the 4th year. By how many feet did the height of the tree increase each year?

answer: 2/3

what does it meant when it says "the tree was 1/5 taller than it was at the end of the 4th "?

请教思路。谢谢。

设每年增加的constant amount 是 x, 那么，第6年的高度就是 4+6x,  第4年底的高度就是 4+4x。 根据题意，可得（4+6x)-(4+4x)=1/5*(4+4x).  解这个方程，可得：x=2/3

注意，tree 的增长是 by a contstant amount each year.  而不是 by a constant rate.

helenying

意思是第六年末树的高度比第四年的高度高了1/5

sigrid920

我列出了这样的式子, 4(1+r%)^6-4(1+r%)^4=1/5

但是请问怎么样简便运算呢?还是我的算式不对呢?

sigrid920

问再问~~

ll_422

帮你顶, 我的式子和楼主是一样的...

??

zjlssw2

2x=(4+4x)*1/5

x是每年增加的高度，左边等于第六年和第四年的高度差，右边是本来高4米，加上长了4年的高度，然后乘以1/5，不知道对不对？

zhu0115

赞同kevin_chee20

4+6x=6/5(4+4x)  ，x为每年的增加高度

vantasy

3/F confuses the "constant amount" which should be height but not rate.