All combination numbers of r(r≤n) elements taken from n different elements are called combination numbers of r elements taken from n different elements. expressed as

explain

Mathematical significance:

Take the number of all combinations of element 0 from 20 different elements.

3. Calculation formula of combination:

4. Calculation process:

, r=0, n=20 into the formula.

=20! /(0! *(20-0)! )=20! /(0! *20! )= 1/0! = 1