本月寂静167 137

LJWISC1OL · 发表于 2010-12-12 21:53:11

11月15日一战告败后，一直都没怎么用心复习，再弄申请材料了，21号二战。现在研究JJ中

问几道题

167:订购A的人20%订购了B,订购B的25%订购了A，问订购B的人数占订购两者总人数的多少？我算了和JJ不同。。。我算了4，JJ答案0.5。求解答过程

137：三个点A（-2,0）B(2,0),c(0,-2) 求举例线段AB 和 OC上所有点距离小于1的面积。
我想，这个面积不应该是个矩形么？怎么答案有PI? 是不是我哪儿里2了？

sdcar2010 · 发表于 2010-12-12 22:04:15

167:订购A的人20%订购了B,订购B的25%订购了A，问订购B的人数占订购两者总人数的多少？我算了和JJ不同。。。我算了4，JJ答案0.5。求解答过程

category A B
total # a b
buy a a .25b
buy b 0.2a b

So those who buy both A and B : 0.2a = 0.25b
Then a = 1.25b

The percentage of people who buy B = b/(a + b - 0.25b) = b/(1.25 b + b - 0.25b) = 0.5
The tricky part is that the total number of people is not a + b since there are a group of people who buy both A and B. So simply using a + b overcounts that group twice.

You can't get 4 because that means 400% of people buy B.

sdcar2010 · 发表于 2010-12-12 22:06:24

137：三个点A（-2,0）B(2,0),c(0,-2) 求举例线段AB 和 OC上所有点距离小于1的面积。
我想，这个面积不应该是个矩形么？怎么答案有PI? 是不是我哪儿里2了？

For the points A and B and C, 所有点距离小于1的面积 is a circle, half of which is covered by the T shape rectangles. The other half is not. So you need to add 3 half circle whose total area = 1.5 pai.

LJWISC1OL · 发表于 2010-12-12 22:44:55

关于137题，我明白你们要计算的图形的表达，但是题目不是说到线段的距离么？这样计算出来的10+1.5PI 是到点的距离

sdcar2010 · 发表于 2010-12-12 23:01:28

A line is composed of billions of points . . .

In fact, it is the billions of circles which combine together that form the T shape triangles plus the three half circles.

本月寂静167 137

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