数学JJ 324.

littlehua

324. DS题，说m和n均为正整数，且知s=m+n, 知道（s-12)(s-15)=0,且有2（m-1)(n-1)=(m+1)(n+1)问可否求n的值？
1 m=4 or m=5
2 m=3 or m=4
思路：

选B

我怎么认为是A

sdcar2010

2(m-1)*(n-1) = (m+1)*(n+1)
mn-3n-3m+1=0
mn-3*s+1=0
mn=3*s-1=35 or 44

So m, n are either the pair of 5 and 7 or the pair of 4 and 11.

B is right since m could ONLY be 4.  Thus n could ONLY be 11.

A is not right since m could be 4 or 5. Hence n could be 11 or 7. Not sufficient.

littlehua

谢谢，分析的太好了。

mengcherry0410

谢谢NN ， 分析得太好了……