小兰上Ｐ１６３的１９７题

in a certain formula,p is directly proportional to s and inversely proportional to r.if p=1 when r=0.5 and s=2,what is the value of p in terms of r, and s?

A s/r

B r/4s

Cs/4r

D r/s

E 4r/s

答案：Ｃ

小蓝上Ｐ１６３－１９６题

each of the integrs form 0 to 9, inclusive ,is written on a separate slip of bland paper and the ten slips are dropped into a hat. if the slips are then drawn one at a time without replacement, how many must be drawn to ensure that the numbers on two of the slips drawn will have a sum of 10?

A THREE

B FOUR

C FIVE

D SIX

E SEVEN

答案：Ｅ

请各位Ｇ友帮忙

p is directly proportional to s and inversely proportional to r

所以p=k*(s/r)

然后代入3个数进去，k=1/4

所以C

第二题

从最小的开始取，直到取到5，就可以了，比如再取一个6可以和4搭，所以是5+1+1

可是第二个还是不太明白，可不可以把组合种类列出来呢？

１　　　９

２　　　８

３　　　７

４　　　６

不是只有这四种组合么？

第2题解法:

1,9;

2,8;

3,7;

4,6;

在取出的数字中, 以上四组中必须要有一组才能使得存在两个数加起来等于10. 所以至少要取7个数字才行.6个数就不行, 比如 0,5,1,2,3,4.