Addition theorem for mutually exclusive events
If A and B are mutually exclusive (A∩B=∅), then
P(A∪B)=P(A)+P(B).
Interpretation: favorable outcomes for A∪B do not overlap, so their counts add — and under the classical scheme the fractions m/n add as well.
Special case: P(A)+P(A)=1.
Without mutual exclusivity you cannot plug this formula in “as is”
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