MATH 3342 : 3 Axiums of Probability

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By: JohnHobbsIII 93
January 19, 2010 | Mathematics - Sturba-Boatwright
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1) P(A) ≥ 0

2) P(Sample Space) = 1

3) If A and B are disjoint, P(A U B) = P(A) + P(B)

FYI: disjoint means there are no components in common

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Roll a fair die

A = The roll is even = {2, 4, 6}    P(A) = 3/6

B = The roll is at least 4 = {4, 5, 6}   P(B) = 3/6

A U B = {2, 4, 5, 6}   P(A U B) = 4/6

Axium 3 : P(A U B) = P(A) + P(B) is not true because A and B is NOT disjoint.

General formula: for any A, B : P(A U B) = P(A) + P(B) - P(A ∩ B)

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  • P(A) + P(A') = 1
  • 0 ≤ P(A) ≤ 1

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Where do probabilities come from?

1) Experimental results to estimate them

  • survey
  • flipped thumbtacks to count point up

2) Counting equally probable events

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Covered sections 2.1 and 2.2

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