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Bayes' formula is derived by expressing the joint probability of the intersection of events A and B, $P(A \cap B)$, in two different ways using the definition of conditional probability. By equating these two expressions, $P(A|B)P(B)$ and $P(B|A)P(A)$, and then dividing by $P(B)$, we isolate $P(A|B)$ to obtain Bayes' theorem.

Conditions: $P(B) > 0$ to allow division.; Events A and B are defined in the same probability space.

Bayes' formula is derived by expressing the joint probability of the intersection of events A and B, $P(A \cap B)$, in two different ways using the definition of conditional probability. By equating these two expressions, $P(A|B)P(B)$ and $P(B|A)P(A)$, and then dividing by $P(B)$, we isolate $P(A|B)$ to obtain Bayes' theorem.

Conditions: $P(B) > 0$ to allow division.; Events A and B are defined in the same probability space.