攒RP贴！！应要求改为大家攒RP贴 讨论数学问题 帮忙解答或提出疑问 LZ 不是NN 但做最大的努力帮大家解答数学问题。

fanfantastic

啊。。。原来如此，谢谢啦~我还以为能算出具体数，怎么看怎么少条件，谢谢：）

yyx

不客气的~

HSE

49. x^2+ax+b一共有两个正数根，问下面哪个一定成立？
1.a<0
2.b<0
3.ab<0

选1和3，想问3是怎么推的？多谢。

-- by 会员 HSE (2010/10/17 9:38:32)

首先对于一个方程 ax^2+bx+c=0两根之和为-b/a  两根之积为c/a
那么在此题中两根之和为-a  两根之积为b   于是 -a<0 b>0可以有结论了吧

-- by 会员 yyx (2010/10/17 13:59:31)

谢谢解答，有个地方纠正一下，题目说有两个正根，所以两根之和应该是正，推出-a>0，则a<0

yiyisue

A school administrator will assign each student in a group of n students to one of m classrooms.  If 3 < m < 13 < n, is it possible to assign each of the n students to one of the m classrooms so that each classroom has the same number of students assigned to it?
(1)    It is possible to assign each of 3n students to one of m classrooms so that each classroom has the same number of students assigned to it.
(2)    It is possible to assign each of 13n students to one of m classrooms so that each classroom has the same number of students assigned to it.
    
lz威武！！

yyx

手误手误 哈哈

yyx

A school administrator will assign each student in a group of n students to one of m classrooms.  If 3 < m < 13 < n, is it possible to assign each of the n students to one of the m classrooms so that each classroom has the same number of students assigned to it?
(1)    It is possible to assign each of 3n students to one of m classrooms so that each classroom has the same number of students assigned to it.
(2)    It is possible to assign each of 13n students to one of m classrooms so that each classroom has the same number of students assigned to it.
   
lz威武！！

-- by 会员 yiyisue (2010/10/17 22:32:45)

我一下子没思路 这题以前见到过 帮你搜了答案 嘿嘿


(1)如果3n/m 是整数，那么n/m是整数。(2)如果13n/m是整数，那么n/m是整数。(1)肯定不充分，n=21，m=9(2)充分。因为m小于13，而且13是质数，所以13一定和m是互质的，所以如果13n能够被m整除，那么n一定能被m整除。选B。

yiyisue

lz人品极佳！！！祝杀鸡成功！！

crocdile

这个答案是C。。？
设M的平均工资为X D为Y  M有m人，D有km人 根据求k就可以知道人数比例
然后列式： （Xm+Ykm）/(m+km)=5000+X
           （Xm+Ykm）/(m+km)=Y-15000
    2个等式相减得到 Y-X=20000
   然后 将等式1展开可以消去mX   ：Ykm=5000m+5000km+Xkm 再消去m 于是得到了k=5000/(Y-X-5000)

-- by 会员 yyx (2010/10/15 19:17:46)

哈哈，太有爱了

yyx

lz人品极佳！！！祝杀鸡成功！！

-- by 会员 yiyisue (2010/10/17 23:01:40)

谢谢~ 大家一起加油

yyx

这个答案是C。。？
设M的平均工资为X D为Y  M有m人，D有km人 根据求k就可以知道人数比例
然后列式： （Xm+Ykm）/(m+km)=5000+X
           （Xm+Ykm）/(m+km)=Y-15000
    2个等式相减得到 Y-X=20000
   然后 将等式1展开可以消去mX   ：Ykm=5000m+5000km+Xkm 再消去m 于是得到了k=5000/(Y-X-5000)

-- by 会员 yyx (2010/10/15 19:17:46)

哈哈，太有爱了

-- by 会员 crocdile (2010/10/17 23:25:11)

必须有爱~