Question

Explain how the binomial theorem works with Combinations: nCk-1*X^(n-(k-1))*Y^k-1

Answer 1

you want to expand the combination only?

Answer 2

if you want a formula for combination then here you go
\[nCr = \frac{ n!}{ (n-r)!r! }\]

Answer 3

^^ exactly right.

Answer 4

you will find the combinatorics will match the lines of pascals triangle.