There was a previous problem in my homework that basically demonstrated that:
10C7 = 9C6 + 8C6 + 7C6 + 6C6
And our question is:
"Use that fact to derive a summation formula involving expressions nC1."
I'm not entirely sure what this means, but I'm assuming we are to use Sigma. This is what I came up with:
$${}_nC_r = \sum_{i=r - 1}^{n-1} {}_iC_{r-1}$$
I'm not sure if I'm even using legal notation here, so any help would be greatly appreciated.
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