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Answers for “P(A|B) 的公式是什么?”

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Find a method

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Substitute the given numerical values into the rearranged formula P(B|A) = P(B)PP(B)P(A|B)/P(A). For example, if P(B)=1/21P(B) = 1/21, P(A|B) = 4/104/10, and P(A)=24/210P(A) = 24/210, the calculation is (1/21∗4/101/21 * 4/10) / (24/21024/210).

Conditions: The formula used is the rearranged Bayes identity for P(B|A).; The given probabilities must be consistent with the formula's assumptions (e.g., P(A)>0P(A) > 0).

Meet the concept

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The book and magnifying glass icons represent specific events in a probability space, serving as labels for the variables in Bayes' theorem. The formula shown is P(Book|Magnifier) = P(Book)P(Magnifier|Book)/P(Magnifier).

Conditions: The icons are used as event labels in the displayed formula.; No fixed letter-to-icon assignment is imposed across displays in the video.

Understand why

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Bayes' theorem is true because the joint probability of two events can be decomposed in two symmetric ways. The probability that both A and B occur is equal to the probability of A multiplied by the conditional probability of B given A, and also equal to the probability of B multiplied by the conditional probability of A given B.

Conditions: A and B are events in the same probability space.; Both A and B have positive probability for the ordinary conditionals used here.

Find a method

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Bayes' formula is derived by expressing the joint probability of the intersection of events A and B, P(A∩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)P(A|B)P(B) and P(B∣A)P(A)P(B|A)P(A), and then dividing by P(B)P(B), we isolate P(A∣B)P(A|B) to obtain Bayes' theorem.

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

Meet the concept

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The overlap region, representing the intersection A∩BA \cap B, serves as the numerator in the conditional probability formula P(A∣B)=P(A∩B)/P(B)P(A\mid B)=P(A\cap B)/P(B). It identifies the specific outcomes within the restricted sample space (event B) that also satisfy event A.

Conditions: We are analyzing P(A∣B)P(A\mid B) using a Venn diagram.; Event B is the conditioning event.

Find a method

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Bayes' theorem is derived by expressing the joint probability P(A∩B)P(A \cap B) in two different ways using the definition of conditional probability. First, P(A∣B)=P(A∩B)/P(B)P(A \mid B) = P(A \cap B) / P(B) implies P(A∩B)=P(A∣B)P(B)P(A \cap B) = P(A \mid B)P(B).

Conditions: P(B)>0P(B) > 0; P(A)>0P(A) > 0

Meet the concept

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The derivation of Bayes' theorem requires that the conditioning probabilities be nonzero. Specifically, when deriving P(A∣B)P(A|B) from the intersection, we divide by P(B)P(B), so P(B)P(B) must be positive.

Conditions: Deriving Bayes' formula from conditional probability definitions.; P(B)>0P(B) > 0 for P(A∣B)P(A|B) to be defined.; P(A)>0P(A) > 0 for P(B∣A)P(B|A) to be defined.

Understand why

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Swapping the events changes both the numerator and the denominator in the conditional probability formula. P(A∣B)P(A\mid B) uses P(A∩B)P(A \cap B) as the numerator and P(B)P(B) as the denominator, whereas P(B∣A)P(B\mid A) uses P(A∩B)P(A \cap B) as the numerator but P(A)P(A) as the denominator.

Conditions: Events A and B have different marginal probabilities (P(A)≠P(B)P(A) \neq P(B)).; Both events have positive probability.