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How to understand the addition rule and the total probability rule?

幫考網校 2020-11-26 16:23:12
The addition rule and the total probability rule are two important concepts in probability theory.

The addition rule states that the probability of the union of two events A and B is equal to the sum of their individual probabilities minus the probability of their intersection. Mathematically, it can be expressed as:

P(A or B) = P(A) + P(B) - P(A and B)

This rule is used when we want to find the probability of an event that can occur in multiple ways. For example, if we want to find the probability of rolling a 1 or a 2 on a fair six-sided die, we can use the addition rule as follows:

P(rolling a 1 or a 2) = P(rolling a 1) + P(rolling a 2) - P(rolling a 1 and a 2)
= 1/6 + 1/6 - 0
= 1/3

The total probability rule, on the other hand, is used to find the probability of an event based on the probabilities of other related events. It states that the probability of an event A is equal to the sum of the probabilities of A occurring given each possible outcome of another event B, weighted by the probability of each outcome of B. Mathematically, it can be expressed as:

P(A) = ∑ P(A|B) * P(B)

This rule is used when we have multiple events that can lead to the occurrence of the event we are interested in. For example, if we want to find the probability of a student passing a test, we can use the total probability rule as follows:

P(passing) = P(passing|studying) * P(studying) + P(passing|not studying) * P(not studying)
= 0.8 * 0.6 + 0.4 * 0.4
= 0.64

Here, we are considering two events - studying and not studying - and finding the probability of passing given each of these events, weighted by their respective probabilities.
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