希望朋友帮我解这道题

mataoturku

1。Al, Lew and Karen  pooled their funds to buy a gift for a friend. Al contributed 2 dollars less than 1/3 of  the cost of the gift and Lew contributed 2 dollars more than 1/4 of the cost. If Karen contributed the remaining 15 dollars, what was the cost of gift.

2。what is total number of integers between 100 and 200 that are divisible by 3?/  有公式吗？

非常感谢

ldyyy

1.gift costs x, then Al contributed (1/3)x-2, Lew contributed (1/4)x+2, so Karen contributed x-[1/3)x-2]-[(1/4)x+2]=15. You can get x

2. integers that are dividable by 3 is 3n(n is integer). 100<3n<200, then you can know how many integer n 

[此贴子已经被作者于2009-6-11 2:57:45编辑过]

deyu578

You can use this way to get the answer.

An=A1+(N-1)d=>198=102+(N-1)*3=>N=33

mataoturku

谢谢各位朋友，由于我在国外生活，上大学数学免考，可是现在发现自己变成数学白吃了。

很想交你们为朋友。也希望咱们以后能够经常联系。mataoliuxue@msn.com

还有An=A1+(N-1)d=>198=102+(N-1)*3， 还有198 和 102， 还有 n-1，从哪得来的。

谢谢各位。

Iljiajia

兄弟，第二题考的是等差数列，差为3

100到200间，能被3整除的最小数为102，最大为198，所以根据等差数列公式198=102+（n-1）3便可求出n值，n即为数列中所有数字的数目