The sum of cubes of three consective integers is always divisible by:
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Let the three consecutive integers be 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 .
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 ) .
Now for any three consecutive integers n − 1 , n , n + 1 , precisely one of then is divisible by 3 .
Thus 3 n ( n − 1 ) ( n + 1 ) = 3 ∗ 3 m = 9 m for some integer m , and so
3 n 3 + 6 n = 9 m + 9 n = 9 ( m + n ) is divisible by 9 for any integer n .
For sake of completeness, note that 2 3 + 3 3 + 4 3 = 9 9 , which is not divisible by any of 4 , 8 or 1 2 .