[求助]数学问题

lucky2009

涛涛数学5-26

Q26:

If r, s, and t are positive integers, is r + s + t even?

(1)
              r + s is even.

(2)
                  s +
                    t is even.

                  

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

D. EACH statement ALONE is sufficient.

E. Statements (1) and (2) TOGETHER are NOT sufficient.

答案是E。我认为是C.

因为1+2 可以推出r+t is even.

各位xdjm 的意见如何？

lucky2009

Up

chrice1988

If r.s.t are all odd, r+s+t=odd.

If r.s.t are all even, r+s+t=even.   以上均满足 1）。2）条件

mapleleaves

(1)
              r + s is even. 推出r和s 同偶或同奇

(2)
                  s +
                    t is even.  推出s和t 同偶或同奇

１＼当ＲＳ同偶，由（２）则Ｔ也为偶　可推出r + s + t even　成立

２＼当ＲＳ同奇，由（２）则Ｔ也为奇　可推出r + s + t even　不成立

因此　答案Ｅ