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Math · High School

Probability and Combinatorics Facts

Basic probability rules, permutations, and combinations for middle and high school math, distinct from the statistical-inference focus of AP Statistics.

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What is the probability of an event?
The ratio of favorable outcomes to total possible outcomes, assuming all outcomes are equally likely.
What is the range of a probability value?
Between 0 and 1, inclusive. P(event) can equal 0 (impossible) or 1 (certain).
What does the complement of an event mean?
The complement of event A is all outcomes that are not in A, denoted A' or A^c.
State the complement rule for probability.
P(A') = 1 - P(A), where A' is the complement of event A.
When are two events mutually exclusive?
When they cannot occur at the same time; they have no outcomes in common.
State the addition rule for mutually exclusive events.
P(A or B) = P(A) + P(B), when A and B cannot occur together.
State the general addition rule for any two events.
P(A or B) = P(A) + P(B) - P(A and B).
When are two events independent?
When the occurrence of one event does not affect the probability of the other.
State the multiplication rule for independent events.
P(A and B) = P(A) x P(B), when A and B are independent.
Define conditional probability.
The probability of an event A occurring given that event B has already occurred, denoted P(A|B).
State the conditional probability formula.
P(A|B) = P(A and B) / P(B), provided P(B) > 0.
Define factorial notation.
n! (n factorial) is the product of all positive integers from 1 to n. Example: 5! = 5 x 4 x 3 x 2 x 1 = 120.
What is 0! (zero factorial)?
By definition, 0! = 1.
Define a permutation.
An arrangement of objects where order matters. The number of permutations of n objects taken r at a time is nPr.
State the permutation formula.
nPr = n! / (n-r)!, where n is the total number of objects and r is the number being arranged.

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