[求助]请教GMAT Prep里的一道数学题

geniuswu06

The rate of a certain chemical reaction is directly propotional to the square of the concerntration of chemical A present and inversely propotional to the concerntration of chemical B present. If the concertration of chemical B is increased by 100 percent ,which of the following is closest to the percent change  in the concerntration of chemical  A required to keep the reaction rate unchanged?

(A) 100% decrease

(B) 50% decrease

(C) 40% decrease

(D) 40% increase

(E) 50% increase

魔方

我的理解是：

设Ａ需要增长Ｘpercent以保持在Ｂ增长的时候此物质不变.

题目的意思是这个化学物质同Ａ平方成正比例，同Ｂ成反比例，列成等式是Ａ＾２／Ｂ

A^2(1+x%)/B(1+100%)=A^2/B

算出来Ｘ等于１，也就是100%.

不知道这样是不是对的．请指点．

看海的猫

设k=m*A^2/B, B增长100%的时候K保持不变，即K=m*[A(1+X)]^2/2B=m*A^2/B,得出：（1+X）^2/2=1, 解出，x=根号2 - 1，约等于0.4，选D