here is a probability question:

if two of the four expressions (x+y), (x+5y), (x-y) and (5x-y) are selected at random, what is the probability of their product being of the form x^2-(by)^2, where b is an integer? thanks in advance

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- October 20th 2006, 04:54 PMhelpjoeplease help
here is a probability question:

if two of the four expressions (x+y), (x+5y), (x-y) and (5x-y) are selected at random, what is the probability of their product being of the form x^2-(by)^2, where b is an integer? thanks in advance - October 20th 2006, 06:17 PMtopsquark
Probability is not my forte so someone please correct me if I'm wrong.

1.

2.

3.

4.

5.

6.

These are all the*different*possibilities. Of these only one pair produces the required form, #2. So we need to pick two choices, of which there are only two elements we can choose. So by the counting principle we have 2 choices for the first factor, then we are left with only 1 possibility for the second choice. There are 4*3 ways to make a product of any two of the factors. So:

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So we have a 1 in 6 chance of getting the required product.

-Dan