两道prep数学题，谢谢

mdiandian

就是琢磨不明白，请帮忙讲讲相关原理，万分感谢！

第一题

the cost of a square slab is proportional to its thickness and also proportional to the square of its length.what is the cost of a sqare slab that is 3 meters long and 0.1 meter thick?

1. the cost of a square slab that is 2 meters long and 0.2 meter thick is $160 more than the cost of a square slab that is 2 meters long and 0.1 meter thick.

2. the cost of a square slab that is 3 meters long and 0.1 meter thick is $ 200 more than the cost of a square slab that is 2 meters long and 0.1 meter thick.

第二题

the rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A preaent and inversely proportional to the concentration of chemical B present. if the concentration of chemical Bis increased by 100 percent,which of the following is closest to the percent change in the concentration 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

mdiandian

第一题答案D，why?

ybalawan

第一题:

设价格为P

1)列方程可以求出P来.

2)同1条件.

ybalawan

第2题:

根据题意,列2个相同的ratio一比就出来了.

mdiandian

谢谢，我真是太笨了！不过关于反比例、正比例的这一类型题，prep里好像只有这两道，偏偏我就是对这种很晕。像那些百分之多少，几分之几的题我作起来都没什么障碍。不知是不是我印象中的没有学过这一部分的事确实的。陈向东的书也没讲过这类题。

能不能拜脱大N再详细地解释一下呢？非常非常感谢！

mdiandian

基本明白了。我的错误在于第二题总觉得是rate=A^2=1/B,所以就是算不出来正确的结果。

现在是不是可以这样理解：

第一题，价格P=xT(thick)*yL^2(length的平方)，根据给的条件解出xy的值再带回去。

第二题，rate=A^2/B,当B增至2B时，A^2/B=(xA)^2/2B,解出x=1.414,即A增40%左右。

请评价对否!