求救--帮忙解决下面这道题

besszhao

在GMAT PREP里碰到这道题，不知道怎么做，随便猜了一个也错了，希望大家帮忙

sysxy1987

图片看不到，重新上传下吧！

besszhao

The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present, If the concentration of chemical B is increased by 100 percent, which of the following is closet to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

 

 

[此贴子已经被作者于2009/9/22 10:44:01编辑过]

besszhao

选择的有

100% decrease

50% decrease

40% decrease

40% increase

50% increase

besszhao

没人帮忙么

sysxy1987

说一个chemical reaction和A的浓度的平方成正比，和B的浓度成反比

B上升了100%，问下面哪个最接近A的浓度要变化使得chemical reaction保持不变。

应该是40% increase吧，对吗？

[此贴子已经被作者于2009/9/22 11:28:02编辑过]

besszhao

对的，答案是这个，你能否把公式列出来啊

besszhao

x2/y=(7X/5)2/2Y?

好像比较接近，我一直在求准确数据，晕死

besszhao

还有一个题，谢谢了

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 6^th year, the tree was 1/5 taller than it was at the end of the 4^th year, By how many feet did the height of the tree increase each year?

正确答案是2/3，
        但我怎么做出来是2/5，
        错在哪里呢？

 

sysxy1987

chemical reaction*KA^2/MB

(K,M代表未知倍率)

现在B变成两倍了，那么A'^2就该变成A^2的两倍啊，大约变成1.4A