# Probability formula Question

1. Jan 26, 2013

### GreenPrint

1. The problem statement, all variables and given/known data

Show that the formula: P(AΔB) = P(A) + P(B) - 2P(A$\bigcap$B)

2. Relevant equations

3. The attempt at a solution

P(AΔB) = P(A) + P(B) - 2P(A$\bigcap$B)

P(AΔB) = P(A$\bigcap B^{c}$)$\bigcup (A^{c}\bigcap B)$

I don't know where to go from here. Thanks for any help.

2. Jan 26, 2013

### Staff: Mentor

Show that the formula does what?
Definition of P(AΔB), perhaps?

3. Jan 26, 2013

### tiny-tim

Hi GreenPrint!

Hint: how would you prove area(AΔB) = area(A) + area(B) - 2area(A$\bigcap$B) ?

4. Jan 26, 2013

### Ray Vickson

Use Venn diagrams; that's their purpose.