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(Choosing vs leaving)
Choosing k items from n is the same as choosing which n-k items to leave behind. C(n,k) = C(n,n-k).
Example: C(5,2) = C(5,3) = 10. Choosing 2 from 5 is equivalent to choosing which 3 to exclude.
This property lets you compute the smaller of k or n-k. C(100,98) = C(100,2) = 4950, much easier to calculate.
