In fact A or B can be rearranged as three mutuall exclusive events
A or B = A and not B or not A and B or A and B
P(A or B)=P(A and not B)+P(not A and B) + P(A and B)
=P(A)-P(A and B)+P(B)-P(A and B)+P(A and B)
=P(A)+P(B)-P(A and B)
A Venn diagram can illustrate logical relation better.
The area represented by Venn digram represent probablity