Permutations and Combinations

Permutations and Combinations :

Permutations

1.nPr = n!/(n-r)!

2.nPn = n!

3.nP1 = n

Combinations

1.nCr = n!/(r! (n-r)!)

2.nC1 = n

3.nC0 = 1 = nCn

4.nCr = nCn-r

5.nCr = nPr/r!

Number of diagonals in a geometric figure of n sides = nC2-n