mathematical induction

By the principle of mathematical induction, the proposition P(n)

1*2+2*3+3*4+...+n(n+1) = 1/3 *n (n+1)(n+2)

when n=k+1

LHS = 1*2+2*3+...+k(k+1)+(k+1)(k+2)

=1/3 *k (k+1)(k+2)+(k+1)(k+2)

and so on

請問為何1*2+2*3+...+k(k+1)+(k+1)(k+2) 會變成1/3 *k (k+1)(k+2)+(k+1)(k+2)
的呢?

當中的 "..."怎樣solve呢
謝謝