Probability Formulas Reference Sheet
Probability helps us measure how likely events are to happen. The basic formula for finding the probability of an event A is P(A) = n(A)/n(U), which compares the number of favorable outcomes to the total possible outcomes.
When working with multiple events, we need specific formulas. For complementary events (something either happens or doesn't), we use P(A) + P(A') = 1. For combined events, the formula P(A ∪ B) = P(A) + P(B) - P(A ∩ B) accounts for potential overlap.
Mutually exclusive events can't happen at the same time, simplifying our formula to P(A ∪ B) = P(A) + P(B). When events might influence each other, we use conditional probability: P(A|B) = P(A ∩ B)/P(B). If events don't affect each other, they're independent, and P(A ∩ B) = P(A)P(B).
💡 Remember that the union symbol (∪) represents "OR" situations, while the intersection symbol (∩) represents "AND" situations when calculating probabilities.
