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Unformatted text preview: ENGRD 2700, Spring 09 Homework 3 Solutions Homework 3 Solutions Problem 1 When two dice are rolled, there are 36 possible outcomes, which can be repre sented as ordered integer pairs ( a,b ) with 1 a,b 6. (a) We need to check if P ( A B 1 ) = P ( A ) P ( B 1 ). We have A = { (4 , 1) , (4 , 2) , (4 , 3) , (4 , 4) , (4 , 5) , (4 , 6) } , B 1 = { (1 , 2) , (2 , 2) , (3 , 2) , (4 , 2) , (5 , 2) , (6 , 2) } , A B 1 = { (4 , 2) } . Since each of the 36 outcomes is equally likely to be realized, we have P ( A ) = 6 1 / 36 = 1 / 6 , P ( B 1 ) = 6 1 / 36 = 1 / 6 , P ( A B 1 ) = 1 / 36 . Thus we see that P ( A B 1 ) = 1 / 36 = P ( A ) P ( B 1 ), which means that A and B 1 are independent. (b) B 2 = { (1 , 2) , (2 , 1) } . Clearly, A B 2 = , which means that A and B 2 are disjoint. (c) We have P ( A ) = 1 / 6 , P ( B 2 ) = 2 / 36 = 1 / 18 , P ( A B 2 ) = P ( ) = 0. Therefore, P ( A B 2 ) = 0 6 = 1 / 108 = P ( A ) P ( B 2 ), so A and B 2 are not inde pendent. (d) B 3 = { (3 , 6) , (6 , 3) , (4 , 5) , (5 , 4) } . So A B 3 = { (4 , 5) } 6 = , which means that A and B 3 are not disjoint. (e) We have P ( A ) = 1 / 6 , P ( B 3 ) = 4 / 36 = 1 / 9 , P ( A B 3 ) = 1 / 36. There fore, P ( A B 3 ) = 1 / 36 6 = 1 / 54 = P ( A ) P ( B 3 ), so A and B 3 are not independent. (f) B 4 = { (1 , 6) , (6 , 1) , (2 , 5) , (5 , 2) , (3 , 4) , (4 , 3) } . So A B 4 = { (4 , 3) } 6 = , which means that A and B 4 are not disjoint. (g) We have P ( A ) = 1 / 6 , P ( B 4 ) = 6 / 36 = 1 / 6 , P ( A B 4 ) = 1 / 36. Therefore, P ( A B 4 ) = 1 / 36 = P ( A ) P ( B 4 ), so A and B 4 are independent. (h) If two events are not disjoint, then they may or may not be independent. If two events are not independent, then they may or may not be disjoint. In short, independence and disjointness are different things. Problem 2 When a fair coin is flipped three times, there are 2 3 = 8 possible outcomes, 1 ENGRD 2700, Spring 09 Homework 3 Solutions which can be represented as threeletter sequences consisting of H s and T s....
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This note was uploaded on 03/05/2009 for the course ENGRD 2700 taught by Professor Staff during the Spring '05 term at Cornell University (Engineering School).
 Spring '05
 STAFF

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