Prove-or-give-a-counter-example-a-1-n-1-k-1-n-k-1-n-1-a-k-1-r-n-n-r-n-r-

Question Number 126935 by Raxreedoroid last updated on 25/Dec/20

P r o v e o r g i v e a c o u n t e r e x a m p l e :

{(a + 1)}^{n - 1} = \underset{k = 1}{\sum^{n}} (\underset{k - 1}{\overset{n - 1}{}}) a^{k - 1}

(\underset{r}{\overset{n}{}}) = \frac{n!}{r! (n - r)!}