GMAT-Prep里面的数学题目， 请帮助

zcooles

1.      When acertain tree was first planted, it was 4 feet tall, and the height of the treeincreased by a constant amount each year for the next 6 years. At the end ofthe 6th year, the tree was 1/5 taller than it was at the end of the4th year. By how many feet did the height of the tree increase eachyear?
A)     3/10             B)2/5                  C)1/2                  D)2/3                 E)6/5



2.  If $1,000 isdeposited in a certain bank account and remains in the account along with anyaccumulated interest, the dollar amount of interest, L, earned by the depositin the first n years is given by the formula L=1000*（（1+r/100）^n-1）. Where r percent is the annual interest rate paid by thebank. Is the annual interest rate paid by the bank greater than 8 percent?
1)      The deposit earns a total of$210 in interest in the first two years
2)      (1+r/100)^2>1.5

sumerlaws

同问~~第一题遇到2次都没想通答案。。。。

daoyinger

First one:
year   1st      2nd     3rd      4th      5th     6th
4feet, 4+a,    4+2a,  4+3a, 4+4a, 4+5a, 4+6a
(4+4a)(1+1/5)=(4+6a)
so a= 2/3

second,
is the answer A?

zcooles

多谢，

恩，第二题是A，怎么解的呢？

aobailing

第二题，（1）将8%代入计算，利息低于210，所以r>8%。
（2）r有两种结果，不确定。

xiaotangyu

The size of a television screen is given as the length of the screen's diagonal. If the screens were flat, then the area of a square 21-inch screen would be how  many square inches greater than the area of a square 19-inch screen?

答案是40，如何计算？

多谢！

dongstar

这道题的意思是电视的size给出的数据其实就是对角线的长度 例如  a square 21-inch screen   那个这个正方形的对角线为21
  设21的边长为x   那么 x^2+x^2=21^2  面积就是x^2
      19             y            y^2+y^2=19^2               y^2
   所以 最后让求的就是x^2-y^2=?    楼主应该会算了吧

aobailing

原来是正方形啊，我以为是长方形呢。

bettyofariN

2)(1+r/100)^2>1.5
这个怎么能不确定呢?
用脑袋想想 1.5拆开根号 也不可能负号阿?  r的值应该唯一阿 如果是 负根号1.5 代入得话 r的值不是负200多了 怎么可能啊?这是利息阿