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 A Difference And Quotient Problem (Posted on 2007-04-04)
Determine analytically all pairs of positive rational numbers (p, q) satisfying p-q = p/q such that p+q is a positive integer.

 See The Solution Submitted by K Sengupta Rating: 3.0000 (1 votes)

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 Got it, I think! Comment 2 of 2 |
If p and q are rational, and p + q is an integer, then they must be of the form p = a/d, q = b/d

Substituting and solving for a yields

a = b*b/(b-d)

b-d must divide b evenly if a is an integer, so let
b-d = k, and b = ek, for any integral k> 0, e > 1

then d = b - k = (e-1)*k

Subsituting into p = a/d, q = b/d

Gives :
p= e*e/(e-1) and q = e/(e-1) for all integral e > 0

But p + q must be integral

P + q = e*(e+1)/(e-1)

which is only an integer for integral e if e = 2 or 3

So, I get (4,2) and (9/2,3/2) as the only two solutions

 Posted by Steve Herman on 2007-04-04 17:53:13

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