# conditional probability confusion

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• Jan 31st 2010, 12:45 PM
alexandrabel90
conditional probability confusion
which is false?

im really confused with conditional probability:(

P(A l B ) + P( A' l B') = 1 false?
P(A l B ) + P( A l B') = 1 false? but why?
• Jan 31st 2010, 01:13 PM
Plato
Quote:

Originally Posted by alexandrabel90
which is false?

im really confused with conditional probability:(

P(A l B ) + P( A' l B') = 1 false?
P(A l B ) + P( A l B') = 1 false? but why?

Both of those are false.
It is true that $P(A|B)+P(A^c|B)=1$
That is easy to see because $P(A\cap B)+P(A^c \cap B)=P(B)$