Symmetry
Pascal's Triangle illustrates the symmetric nature of a combination. C(n,r) = C(n,n-r)
Example: C(10,4) = C(10,6) or C(100,99) = C(100,1)
Shortcut formula for finding a combination
Since combinations are symmetric, if n-r is smaller than r, then switch the combination to its alternative form and then use the shortcut given above.
C(n,r) = first r factors of n factorial divided by the last r factors of n factorial
