.Let and be integers such that and
Find the number of unordered pairs such that .
Definition: "Unordered" pairs means that and are the same. Count that as one pair, not two.
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We don't really need to list all the pairs . Let's calculate the total number of pairs first .
A × B = 3 6 0 = 2 3 × 3 2 × 5 .
Total pairs = ( 3 + 1 ) × ( 2 + 1 ) × ( 1 + 1 ) = 2 4 . These are all the ordered pairs with positive integer A and B . Unordered pairs will be 2 2 4 = 1 2 . Out of these 1 2 pairs 6 of them will have A < 6 . Total Pairs left = 6 . Since negative integers are also included , total pairs are 6 × 2 = 1 2