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How to proof P(A U B U C) without using Venn Diagram

by ooooo
Tags: diagram, proof, venn
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ooooo
#1
Oct4-08, 04:24 PM
P: 3
Do you know how to proof

P(A U B U C) = P(A) + P(B) + P(C) - P(A^B) - P(B^C) - P(C^A) + P(A^B^C)


^ is intersection.

Do you know how to find P(A U B U C U D)

Thank you very much.
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statdad
#2
Oct4-08, 05:11 PM
HW Helper
P: 1,361
Let

[tex]
D = B \cup C
[/tex]

and note that

[tex]
A \cup B \cup C = A \cup D
[/tex]

then

[tex]
\begin{align*}
\Pr(A \cup B \cup C) & = \Pr(A \cup D)\\
& = \Pr(A) + \Pr(D) - \Pr(A \cap D) \\
& = \Pr(A) + \Pr(B \cup C) - \Pr(A \cap D)\\
& = \Pr(A) + \Pr(B) + \Pr(C) - \Pr(B \cap C) - \Pr(A \cap D)
\end{align*}
[/tex]

The rest of the proof comes from realizing that

[tex]
\Pr(A \cap D) = \Pr(A \cap \left(B \cup C\right)) = \Pr((A \cap B) \cup (A \cap C)),
[/tex]

using the Addition Rule for probability to expand the final term, and being very careful with positive and negative signs.
ooooo
#3
Oct4-08, 07:00 PM
P: 3
Thank you so much Statdad. I would like to ask another question.

How to proof P(A U B) = P(A) + P(B) - P(A ^ B) ?

Thank you again.

statdad
#4
Oct4-08, 07:02 PM
HW Helper
P: 1,361
How to proof P(A U B U C) without using Venn Diagram

This proof isn't needed for the problem you posted above - is there a reason you need it here?
ooooo
#5
Oct4-08, 07:06 PM
P: 3
Sorry. I'm just curious. :)
statdad
#6
Oct4-08, 07:14 PM
HW Helper
P: 1,361
No - I was interrupted by someone at the door.
Here is one method - there are others.
First, note that

[tex]
A \cup B = (A-B) \cup (A \cap B) \cup (B - A)
[/tex]

and the three sets on the right are pair-wise disjoint. Now

[tex]
\begin{align*}
\Pr(A \cup B) & = \Pr(A-B) + \Pr(A \cap B) + \Pr(B - A)\\
& = \left(\Pr(A-B) + \Pr(A \cap B) \right) + \left(\Pr(B-A) + \Pr(A \cap B)\right) - \Pr(A \cap B) \\
& = \Pr(A) + \Pr(B) - \Pr(A \cap B)
\end{align*}
[/tex]

Again, sorry for the abrupt end to my previous post - I'm getting really tired of our election season.


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