求教一道pp题，谢~

aprilhuang

368.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 the chemical B present . If the concentration of chemical B is increased by 100 persent, which of the following is closest to the percent change in the concentration of chemical of A required to keep the reaction rate unchanged?

A 100% increase

B 50% decrease

C 40% decrease

D 40 % increase

E 50% increase

A 100% increase

B 50% decrease

C 40% decrease

D 40 % increase

E 50% increase

A 100% increase

B 50% decrease

C 40% decrease

D 40 % increase

E 50% increase

QUOTE:

还是不太理解答案是D,以前讨论好像也没说明白，望达人指点！

windlake

你这样想。因为增加了100%，就是说变成了200%，如果A浓度不变，那么本来rate就应该相应变成了原来的1/2。那么要想rate不变，A就必须增加，多少呢？因为题目说是和sqare成正比，所以应该增加为原来的根号(2)倍，那么就会抵消那个降低的1/2了。根号2就是1.414，那么就是增加40%。

XIAOYUE78

是不是选D啊,

题目说a certain chemical reaction(设为X)与A的平方成正比, 与B成反比, 根据题意可列方程,

x=(A的平方)/B

当B增加100%时, 方程变为(A的平方)/2B, 要使原来的方程不变, 那么分子就必须变为2*(A的平方) 因为

(1.4A)的平方=1.96(A的平方) 

问题是closest to the percent ,1.96 ≈   2, 所以选D

Yingyutao22

多谢多谢，理解了，square和by 100%都没有注意到。还是靠仔细亚，Thx a lot!!

aprilhuang

many  thanks !!