[求解] PREP－PS-80 -大家帮看看

yayafl · 发表于 2011-1-8 03:54:20

80. 9169-!-item-!-187;#058&006279
If r and s are integers, and rs + r is odd, which of the following must be even?

(A) r
(B) s
(C) r + s
(D) rs - r
(E) r2 + s

不活了。超级简单的一个 PS就出不来了。求解。谢谢大家

sdcar2010 · 发表于 2011-1-8 04:12:49

rs + r = 2n + 1
r(s+1) = 2n +1
So both r and (s+1) is odd!

So s must be even.

Then r*2 + s must be even.

Choose B and E.

yayafl · 发表于 2011-1-8 04:44:50

rs + r = 2n + 1
r(s+1) = 2n +1
So both r and (s+1) is odd!

So s must be even.

Then r*2 + s must be even.

Choose B and E.

-- by 会员 sdcar2010 (2011/1/8 4:12:49)

牛牛，谢谢了。那就是说题本身有问题了。呵呵。我就不想了。考试的时候不会有这样的。

[求解] PREP－PS-80 -大家帮看看

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