
Ms. Garcia Math 


Probability 
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The probability of event A is the number of ways event A can occur divided by the total number of possible outcomes. 






Events mutually exclusive 
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Two events are mutually exclusive if, when one event occurs, the other cannot, and vice versa. 


Example: Tossing a coin once. Can result in either heads or tails, but not both. 




Complementary event 
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The complement of any event A is the event [not A] 


Example: Turning a Head and Turning a Tail are complementary events. 




P(A) + P(Not A) = 1 
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The probabilities of two complementary events add up to 1 . 
P(Head) + P(Tail) = ½ + ½ = 1 




Simple event 
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An event that cannot be decomposed is called a simple event. 


Example: rolling a 6 on a die has the probability of 1/6 




Compound event 
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Are the combination of multiple simple events 


Example: rolling an even number on a die has the probability of 3/6 

P(even) = P(2 or 4 or 6) = 3/6 = 1/2 




Adding probabilities 
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If two events have no outcomes in common, the probability that one or the other occurs is the sum of their individual probabilities. 


P(2 or 4 or 6) = 1/6 + 1/6 + 1/6 = 3/6 






