Complement of an event
The complement of an event is the event that the original event does not happen.
Practice This ConceptMain explanation
Teacher explanation
If the probability of an event is known, its complement helps us find the remaining chance. The two probabilities always add to 1. This is very useful when the direct counting is longer than counting the opposite case.
Example
If the probability of getting a head on a fair coin is 1/2, then the probability of not getting a head is also 1/2.
Simple analogy
Complement means the rest, so subtract from 1.
Common confusion
Students often subtract from the wrong number or confuse the complement with a completely different event.
Exam tip
Use complement when the opposite event is easier to count than the original event.
Answer writing and exam use
1-mark use
Write the exact meaning of complement of an event in one clean line.
2-mark use
Define complement of an event and add one example or condition.
3-mark use
Explain complement of an event, show the method or example, and mention the common mistake.
Practice this concept with focused MCQs
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