# Math Help - conditional probability confusion

1. ## 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?

2. 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 $P(A\cap B)+P(A^c \cap B)=P(B)$