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Harvard-MIT Math Tournament

Number theory Difficulty 4.8 AIME Prove it United States

Problem:
Find the 6-digit number beginning and ending in the digit 2 that is the product of three consecutive even integers.

Solution

Solution:
Because the last digit of the product is 22, none of the three consecutive even integers end in 00. Thus they must end in 2,4,62, 4, 6 or 4,6,84, 6, 8, so they must end in 4,6,84, 6, 8 since 2⋅4⋅62 \cdot 4 \cdot 6 does not end in 22. Call the middle integer nn. Then the product is (n−2)n(n+2)=n3−4n(n-2) n (n+2) = n^{3} - 4n, so n>2000003=200⋅1033≈60n > \sqrt[3]{200000} = \sqrt[3]{200 \cdot 10^{3}} \approx 60, but clearly n<3000003=300⋅1033<70n < \sqrt[3]{300000} = \sqrt[3]{300 \cdot 10^{3}} < 70. Thus n=66n = 66, and the product is 663−4⋅66=28723266^{3} - 4 \cdot 66 = 287232.

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