One math question

EllenTony · 发表于 2010-6-20 03:54:41

If x and y are positive integers and 1 + x + y + xy = 15, what is the value of x + y?

A.3
B.5
C.6
D.8
E.9

answer is C
Is there another way to get the answer other than trying to put every answer into the equation? Thanks.

yizhutou1 · 发表于 2010-6-20 05:29:02

因式分解得 (x+1)(y+1)=15

又因为 x,y 均为正整数所以 15=3*5 (若15=1*15，x和y中必有一个等于0 不合题意舍去)

所以 x+1=3 y+1=5, (本题中 x,y无差异因为求的是x+y的值)

解出来 x=2 y=4

和为6咯~

EllenTony · 发表于 2010-6-21 04:19:57

wonderful solution! Thanks a lot.

One math question

所属分类: GMAT考试

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