Combinations, and Problems involving Combinations

• A combination is a distinct group (or set) of objects without regard to their arrangement.
  • _nC_r = n!/[(n-r)!r!]
• Consider the set A = {a,b,c}
  • All two letter permutations (₃P₂ =6/1 = 6 of them)
  • ab, ac, ba, bc, ca, cb
  • All two letter combinations (₃C₂ =6/2 = 3 of them)
  • ab, ac, ba, bc, ca, cb
• Do some problems on page 749
• Do some problems on page 755