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permution and combination

n! which reads n-factorial and it is interpreted thus
n!= n(n-1)(n-2)(n-3)! By implication we could simply say multiply down.
for instance 5!= 5 * 4 * 3 * 2 * 1= 120 and 3!= 3 * 2 * 1= 6
NB: 0! and 1! are both 1. It is fixed. This knowledge of factorial will help us as we progress in today's class. but first try these
(i) 6! (ii) 8! (iii) 4! + 5!
nPr = n!/(n-r)! this represent permutation which literarily mean arrangement.
5P3=5!/(5-3)! = 5!/2!= 5 * 4 * 3 * 2!/2! = 5 * 4 * 3 = 60
Ex 1. How many 3 letter words can we make with the word love?
Solution
4P3= 4!/(4-3)! = 4 * 3 * 2 * 1!/ 1! = 24
Ex 2. How many 4 letter words can we make out of the word DEMONS?
Solution
6P4= 6!/(6-4)! = 6 * 5 * 4 * 3 * 2!/2!= 6 * 5 * 4 * 3= 360

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