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Unformatted text preview: ISYE2028A and B: Probability Lecture 2 Dr. Kobi Abayomi February 10, 2009 1 The basic principles of counting It is fitting that a probability class begins with counting, for a probability of an event may be seen as an enumeration of the ratio: number of possible ways for the event to happen to number of possible ways for anything to happen. The counting principle is: If an event can happen in any of n ways, and when the event has occurred, another event can happen in any of m ways, then the number of ways in which both events can happen is nm . Example : If there are 3 candidates for governor and 5 for mayor, the two offices can be filled in 3 5 = 15 ways Example : How many functions defined on n points are possible if each functional value is either 0 or 1: 2 n Example : How many different 7 place license plates are possible if the first 3 places are letters and the final 4 numbers: 26 3 * 10 4 . 1.1 Factorial n Factorial n , denoted by n !, is defined as: n ! = n ( n 1)( n 2) 1 Thus 5! = 5 4 3 2 1 = 120.The factorial notation is very useful: 1 Example : How many different words can be made with using each of the letters a,b,c,d,e only once: Pick the first letter  5 ways to do it; pick the second letter  4 ways,...pick the last letter 1 choice left 5 4 3 2 1. Which brings us to... 1.2 Permutations A permutation of n different objects taken i at a time is an arrangement of i out of the n objects, with attention given to the order of arrangement . We denote the number of ar rangements of n objects taken i at a time as: P n r = n ( n 1) ( n i + 1) = n ! ( n i )! Permutations arise naturally in settings where order matters. If you wanted to know the number of possible words of length four in the English alphabet you would: P 26 4 = 26! (26 4)! = 26! 22! = 26 25 24 23 = 358800 Wow! A way to think about the permutation equation: If we were to count the total of number of ways to arrange n distinct objects, we would have n !. When we only look at i out of the n objects, we (basically) count the total number  n !  and then divide the overcount  ( n i )! out. The number of permutations of n objects consisting of k groups of which n 1 are alike, n 2 are alike, etc. is n ! n 1 ! n 2 ! n k . For example, the number of permutations of letters in the word statistics: 10! 3!3!1!2!1! = 50400, since there are 3ss, 3ts, 1a, 2is, and 1c. Example : How many different letter arrangements can be formed using the letters in the word PEPPER : 6! 3!2! = 60. 2 Combinations, Binomial Theorem, Multinomials A combination of n different objects taken i at a time is a selection of i out of the n objects, with no attention given to the order of arrangement . Our notation here is: 2 C n i = n ( n 1) ( n i + 1) i !...
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This note was uploaded on 11/08/2009 for the course ISYE 2028 taught by Professor Shim during the Spring '07 term at Georgia Institute of Technology.
 Spring '07
 SHIM

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