Here is a good Venn diagram site
with the basics for review:
Basics about Venn diagrams

Rules for set manipulation:

(a) Distributive Rule:
.

(b) De Morgan's Laws:
.

More generally
.

and for a definitely more colorful set of web pages about Venn diagrams please click Here is a sophisticated Venn site

Events are said to be mutually
exclusive if

Note that this is a property of the sets, not of the probability.