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A Probability Course for the Actuaries A Preparation for Exam P/1

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22 BASIC OPERATIONS ON SETS<br />

Solution.<br />

Using a Venn diagram one can easily see that [(A∪B)∩(A∪C)∩(B c ∩C c )] =<br />

A − [A ∩ (B ∪ C)]<br />

If A ∩ B = ∅ we say that A and B are disjoint sets.<br />

<strong>Exam</strong>ple 2.6<br />

Let A and B be two non-empty sets. Write A as <strong>the</strong> union of two disjoint<br />

sets.<br />

Solution.<br />

Using a Venn diagram one can easily see that A ∩ B and A ∩ B c are disjoint<br />

sets such that A = (A ∩ B) ∪ (A ∩ B c )<br />

<strong>Exam</strong>ple 2.7<br />

Each team in a basketball league plays 20 games in one tournament. Event<br />

A is <strong>the</strong> event that Team 1 wins 15 or more games in <strong>the</strong> tournament. Event<br />

B is <strong>the</strong> event that Team 1 wins less than 10 games. Event C is <strong>the</strong> event<br />

that Team 1 wins between 8 to 16 games. Of course, Team 1 can win at most<br />

20 games. Using words, what do <strong>the</strong> following events represent<br />

(a) A ∪ B and A ∩ B.<br />

(b) A ∪ C and A ∩ C.<br />

(c) B ∪ C and B ∩ C.<br />

(d) A c , B c , and C c .<br />

Solution.<br />

(a) A ∪ B is <strong>the</strong> event that Team 1 wins 15 or more games or wins 9 or less<br />

games. A ∩ B is <strong>the</strong> empty set, since Team 1 cannot win 15 or more games<br />

and have less than 10 wins at <strong>the</strong> same time. There<strong>for</strong>e, event A and event<br />

B are disjoint.<br />

(b) A ∪ C is <strong>the</strong> event that Team 1 wins at least 8 games. A ∩ C is <strong>the</strong> event<br />

that Team 1 wins 15 or 16 games.<br />

(c) B ∪ C is <strong>the</strong> event that Team 1 wins at most 16 games. B ∩ C is <strong>the</strong><br />

event that Team 1 wins 8 or 9 games.<br />

(d) A c is <strong>the</strong> event that Team 1 wins 14 or fewer games. B c is <strong>the</strong> event that<br />

Team 1 wins 10 or more games. C c is <strong>the</strong> event that Team 1 wins fewer than<br />

8 or more than 16 games

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