Respuesta :

malakessa224

Step-by-step explanation:

1.2+2.3+3.4+⋯+n(n+1)=∑k=1nk(k+1)

=∑k=1n(k2+k)

=∑k=1n(k2)+∑k=1n(k)

=n(n+1)(2n+1)6+n(n+1)2

=n(n+1)6(2n+1+3)

=n(n+1)6(2n+4)

=n(n+1)6[2(n+2)]

=n(n+1)(n+2)3

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