If A and B are events such that P(A|B) = P(B|A), then
Given: P(A|B) = P(B|A) ……….(i)
Now, we know that
By definition of conditional probability,
……….(ii)
……….(iii)
Using (i), we have
P(A|B) = P(B|A)
⇒ P(A) = P(B)