# Math Help - conditional prob

1. ## conditional prob

How do you prove that P(A l B) = 1 - P( A ' l B)?

2. Use P(B)=P(AB)+P(A'B)

3. sorry i just started learning stats and so have never seen this equation before. do you mean
P(B)=P(A)P(B)+P(A')P(B)?

thanks

4. Originally Posted by alexandrabel90
sorry i just started learning stats and so have never seen this equation before. do you mean
P(B)=P(A)P(B)+P(A')P(B)?

thanks

Nope I wrote what I meant.
DRAW a Venn diagram and see if you can convince yourself of my equation.
It's not hard to see and you need it here.
There's no way of getting around it.

YOU can use your equation, but then you need to assume independence
which is not necessary.

5. Some people may not be used to this writing...

$P(AB)$ stands for $P(A\cap B)$ (when talking about events)