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number theory senior
Problem
Let be the product of any three consecutive positive odd integers. The largest integer dividing all such is:
(A)
(B)
(C)
(D)
Solution
Product can be written as ,,. Because is defined as a "3 consecutive odd integer" product impies that must be divisible by at least 3. A is ruled out because factors of 5 only arise every 5 terms, if we were to take the 3 terms in the middle of the factors of 5 we wouldn't have a factor of 5. Obviously B is impossible because we are multiplying odd numbers and 2 would never become one of our prime factors. C is ruled out with the same logic as A. Lastly E is ruled out because we have already proved that 3 is possible, and the question asks for greatest possible. * In first step we could also apply modular arthmetic and get the same answer.
Final answer
D