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How many integers n, with 2≤n≤80, is ((n-1(n)(n+1))/6 equal to an integer?

 Jan 6, 2021
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Out of 3 consecutive numbers, one must be divisible by 2 and one must be divisible by 3 so the product must be divisible by 6. There are 79 integers from 2 to 80. 

:D

 Jan 6, 2021

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