why is that P(A union B) is not always P(A) + P(B)?

Answer 1

P(AUB) = P(A) + P(B) - P(A^B)

the Union = sum of the A and B minus their intersection

if there's no overlap, if they are mutually exclusive,

if no intersection, if on a Venn diagram they are two

entirely separate circles, then P(AUB)= P(A)+P(B)