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number theory intermediate
Problem
How many pairs of positive integers are there such that and have no common factors greater than and: is an integer?
(A)
(B)
(C)
(D)
Solution
Let So , where I is an integer. Algebraic manipulations yield: . The discriminant of this must be the square of a rational number, call this R. So . I is the sum of and . To have an integer sum, and must have the same denominator, namely 3. We proceed with casework. Case 1. , . This yields , which is not an integer. This case produces 0 solutions. Case 2. , . This yields . Substituting into our original equation yields: . Factoring gives: , . This case produces 2 solutions, namely (1,3) and (14,3). Case 3. , . This yields . Substituting into our original equation yields: . Factoring gives: , . This case produces 2 solutions, namely (2,3) and (7,3). Case 4. , . This yields , which is not an integer. This case produces 0 solutions. Altogether, we have 4 solutions, so our answer is .
Final answer
A