N∈ ln
we set a= n(n²+11)
show that a is divisible by 3
( n is natural number )
try to me in this i need it tomorrow

@Konrad509's answer was fantastic. 

What you first need to know is this. 3 consecutive natural numbers multiplied by each other, for instance: (1*2*3) or (2*3*4) or (3*4*5) or (4*5*(2*3)) can be described using the abstract expression:

(n-1)n(n+1)



Now:

A=n(n²+11),

and:

n(n²+11)

=n(n²-1+12)

=n³-n+12n

=n(n²-1)+12n

=n(n+1)(n-1)+12n

=(n-1)n(n+1)+(4*3)n



So:

A=(n-1)n(n+1)+(4*3)n

Therefore, A is divisible by 3.

is the product of 3 consecutive natural numbers, so it has be to divisible by 3.

is also divisible by 3 because 12 is divisible by 3.

If both elements of the sum are divisible by 3 so the sum itself is divisible by 3 as well.