Might want to remember that

Algebra Level 2

The sum of cubes of three consective integers is always divisible by:

12 4 9 8

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2 solutions

Let the three consecutive integers be n 1 , n , n + 1. n - 1, n, n + 1. Then the sum of their cubes is

( n 1 ) 3 + n 3 + ( n + 1 ) 3 = ( n 3 3 n 2 + 3 n 1 ) + n 3 + ( n 3 + 3 n 2 + 3 n + 1 ) = 3 n 3 + 6 n . (n - 1)^{3} + n^{3} + (n + 1)^{3} = (n^{3} - 3n^{2} + 3n - 1) + n^{3} + (n^{3} + 3n^{2} + 3n + 1) = 3n^{3} + 6n.

Next, note that 3 n 3 + 6 n 9 n = 3 n 3 3 n = 3 n ( n 2 1 ) = 3 n ( n 1 ) ( n + 1 ) . 3n^{3} + 6n - 9n = 3n^{3} - 3n = 3n(n^{2} - 1) = 3n(n - 1)(n + 1).

Now for any three consecutive integers n 1 , n , n + 1 , n - 1, n, n + 1, precisely one of then is divisible by 3. 3.

Thus 3 n ( n 1 ) ( n + 1 ) = 3 3 m = 9 m 3n(n - 1)(n + 1) = 3*3m = 9m for some integer m , m, and so

3 n 3 + 6 n = 9 m + 9 n = 9 ( m + n ) 3n^{3} + 6n = 9m + 9n = 9(m + n) is divisible by 9 \boxed{9} for any integer n . n.

For sake of completeness, note that 2 3 + 3 3 + 4 3 = 99 , 2^{3} + 3^{3} + 4^{3} = 99, which is not divisible by any of 4 , 8 4,8 or 12. 12.

Nicolas Bryenton
Dec 9, 2015

Let the first integer be odd. Thus, the second is even and the third odd. We know that cubing an integer preserves parity. Hence, the sum of the cubes of three consecutive integers is odd if the first is odd. Then clearly, the sum of the cubes of three consecutive integers is not always even and will not always be divisible by any even number. Therefore, the answer is 9, since all other options are even.

Multiple choice for the win.

The sum of two odds and an even is even... However, your explanation kind of still stands, as if the first number is even, the sum of the cubes must be odd and thus can't be divisible by any of the options except 9. It's because of this really easy deductive reasoning that I think the question ought to be changed to open-ended.

Chris Callahan - 5 years, 6 months ago

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Oh my gosh, I can't believe I made that error. Well, as you kindly pointed out, my solution slightly restated still does apply. Thanks.

Nicolas Bryenton - 5 years, 6 months ago

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