求助 OG13 DS136

detective

136. If° represents one of the operations +,•-, and x, is
k°(£ +m) =(k ° £)+(k ° m)for all numbers k,£,
andm?
(1) k ° 1is not equal to 1 ° k for some numbers k.
(2) ° represents subtraction.

(1) For operations + andX, k ° 1is equal to
1 ° k since £ + l = l + /£and>£xl = lx/i
Therefore, the operation represented
must be subtraction. From this, it
is possible to determine whether
QQ:1014347461制作
k-(£ +m) =(k-£) +(k-m) holds for all
numbers k, £, and m; SUFFICIENT.
(2) The information is given directly that
the operation represented is subtraction.
Once again, it can be determined whether
k-(t +m) = (k-e) +(k-m) holds for all
numbers k, £, and m; SUFFICIENT
为什么减号可以hold for all numbers
感觉减号的话只能是k=0才成立了
谢谢！

torresAing

没错，减号并不能让等式成立，但是知道这个条件足够你回答这道题了，所以也就是充分的咯~

detective

不太明白~能麻烦再细说吗！

torresAing

额，DS类型题目考的是：“decide whether the data given are sufficient for answering the question" 即给出的两个条件是否足够解答这个问题。。。


这道题问的是 is k°(￡ +m) =(k°￡) +(k°m) or not ，即等式是否成立。

虽然 “°是减号” 的情况下等式不成立，但是你回答了这个问题，即answer the question了，所以都是sufficient的

detective

嗯嗯~明白了！以前题目理解的不大清楚
谢谢指点哈！！！