# Unions and Intersections

1. Oct 9, 2012

### vanitymdl

1. The problem statement, all variables and given/known data
A and B are independent, P(A) = 0.30, P(B) = 0.50. Compute P(A U B) and P(A' intersect B).

3. The attempt at a solution
P(A U B) = 0.30 + 0.50 = 0.80
P(A' intersect B) = 1 - 0.30 = 0.70
this is obviously wrong

2. Oct 9, 2012

### ehild

It is really wrong. What does it mean that A and B are independent? When is it true that P(A U B) = P(A)+P(B)?
Try to draw the Venn diagram.

ehild

3. Oct 9, 2012

### vanitymdl

Well I got the first one incorrect and I don't know exactly why because wouldn't the union be what is included in the set A and B. That's why I added them.

For the second on I'm no sure how to really do that one. I just understand it as not in A and in B

4. Oct 9, 2012

5. Oct 9, 2012

### SammyS

Staff Emeritus
Yes, of course you're correct, ehild !

(I have deleted my earlier post. )

6. Oct 9, 2012

### ehild

P(AUB)=P(A)+P(B)-P(A$\cap$B)
Look after how to calculate the probability P(A$\cap$B) if A and B are independent.

It is the probability that A does not happen and B happens.

ehild