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# Section 5.2 The Addition Rule and Complements

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Section 5.2 The Addition Rule and Complements

All disjoint events in a sample space sum to
KEY IDEA All disjoint events in a sample space sum to 1

Two events are disjoint if they have no outcomes in common.
Disjoint events are also called mutually exclusive events. Example: Toss a coin. H and T are disjoint. Example: Roll a die. 1,2,3,4,5,6 are disjoint. Example: Roll a die. The events E={1,2,3} and F={4,5,6} are disjoint. Example: Roll a die. The events E={1,2,3} and F={2,4,6} are not disjoint. They share {2}.

Venn Diagrams Used to display simple probability logic
Venn Diagrams Used to display simple probability logic. But requires special software to draw on computer.

Let S=sample space={1,2,3,4,5,6,7,8,9,10} A={1,2,3,4,10), B={4,5,6,7,9}

“Union of A and B” = A or B = A U B
= { 1, 2, 3, 4, 5, 6, 7, 9, 10 } “Intersection of A and B” = A and B = A ∩ B = { 4 } “Not A” = complement of A = AC = {5, 6, 7, 8, 9}

Addition Rule for Disjoint Events P(A or B) = P(A U B) = P(A) + P(B) IF A and B are disjoint events

For non-disjoint and disjoint events P(A U B) = P(A) + P(B) – P(A∩B)
Notice 1: P(A)+P(B) counts intersection twice. Notice 2: If A and B are disjoint, P(A∩B)=0.

Complement Rule P(AC)=1-P(A)

Show P(A U B) = P(A) + P(B) – P(A∩B)

P(A and C) = ? P(A or C ) = ? P(A and B) = ? P(A or B) = ? P(A or B or C)=?

From a 52 card deck, what is P( Spades or King)?

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