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Combination with zero
A combination with 0 is
Explanation
The general formula for combinations is
We start with r = 3 and see
There is nothing special about that. We continue with r = 2 and get
That follows the same scheme. Now we take r = 1 and find
For r = 0 it gets
There is no n in it anymore, so it is a constant. It is determined by definition that 0! ≝ 1. Can you now decide that ? We will check that, and find
So, in that case you can also apply the formula.