PP-T2-PS

[Huang Hsin-Yi]

If two of the four expressions X+Y, X+5Y, X-Y, and 5X-Y are chosen at random, what is the probability that their product will be of the form of X^2-(by)^2, where b is an interger?

Ans:1/6

求思路

[sunnyblue13s]

請問...b在何時...會是整數??

(X+Y)(X-Y) - 有沒有很眼熟!!

四個東西"排列" = 母體

求機率= 發生數/母體

[Huang Hsin-Yi]

感謝提點

全部機率為C(4,2)=6種
(X+Y)(X-Y)是唯一滿足 X^2-(by)^2的組合=1種