15-3

Permutations and Combinations
This week I'm going to reteach section 15-3, Permutations and Combinations. In certain situations, the order in which choices are made is important, and other times it is not. When the order is important we use the Permutation formula, and when it is not we use the Combination formula.
-Permutation formula: nPr = n!/(n-r)!
-Combination formula: nCr = n!/(n-r)!r!

Example 1: In how many ways can a club with 13 members choose 4 different officers?
-We will use Permuation because the order matters here.
13P4 = 13!/(13-4)! = 13!/9! = 13 x 12 x 11 x 10 = 17, 160

Example 2: In how many ways can you choose 3 letters from the word LOGARITHM if the order of letters is unimportant?
-We will use Combination because the problem clearly states that the order is unimportant.
9C3 = 9!/(9-3)!3! = 9!/(6)!3! = 9 x 8 x 7/3 x 2 = 84