最初由 叶落无声 发布
第一轮谁都说不出答案,则证明A!=B!=C,或者是B或C的2倍数。
已知C=144,而A能判断自己不是36,就是252。B则可以判断自己不是108就是180。而C在第一轮就可以知道自己不是72就是144。但是如果假设C是72,则B在第一轮就已经知道自己不是36就是108,而如果是36,C在第一轮就知道自己只能是72,但是C没有说出来,所以B是108。
当然这个答案有点反推理的味道。但是如楼上所说,有很多种可能得出144。但是在第二轮就能得出答案的只有36和108。
好, 反推理,美中不足。 为什么 A = 36 ?可正推之:
已知:A>0, B>0, C>0, 均为正整数
A = ( B+C or |B-C|);
B = ( A+C or |A-C|);
C = ( A+B or |A-B|);
第一轮:
A: no ==> B != C
B: no ==> A != C
C: no ==> A != B ==> 2A != B (or 2B != A)
(if 2A = B, and we had C != A, i.e. C != |B-A|, so C=B+A, C解, C:yes)
第二轮:
A: no ==> 2B != C (or 2C != B)
(if 2B = C, and we had A != C, i.e. A != |B-C|,so A=B+C, A解, A:yes)
B: no ==> 2A != C (or 2C != A) 且 C != 3A
(if 2A = C, and we had B != A, i.e. B != |A-C|,so B=A+C, B解, B:yes)
(if 3A = C, and we had B != 2A, i.e. B != |C-A|,so B=A+C, B解, B:yes
, 即B应知道 B=4A)
C: yes ==> C = 144,
we assume B>A first
coz C knows C != 2A by B answered "no", and then C got answer 144.
so 2A and 144 were 2 possible answers for C at the first round.
so C had this 2A possible answer at first round:
so C=|B-A| = 2A (if C= A+B = 2A, it's impoosible, coz A!=B)
so B = 3A
at 2nd round C got C != 2A, so C must be C = A + B = 4A = 144
so A= 36
and B=108
(C != 3A has to be considered too, but 3A is not a possible answer at the frist round, if 3A is a answer, then B will know B=4A at the 2nd round, but B say "no", so C = 5A and C should say a number is equit to 5A. 而144不能为5整除, 因此上说3A不是, 而2A才是C的另一个可能的解B-A )
for A>B also can be solved as above, then we get B =36, A = 108