Maths Olympiad Prep

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HMMT February

Algebra Difficulty 4.5 AIME Find the answer

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

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Because the last digit of the product is 2, none of the three consecutive even integers end in 0. 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 2. Call the middle integer nn. Then the product is (n−2)n(n+2)=n3−4n(n-2) n(n+2)=n^{3}-4 n, 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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