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A compound event contains one or more of the outcomes in the sample space. The outcomes that are left are read as “not ”, and can be written as . and with a bar on top (“A Bar”) are then complementary events. So you know that:
Rule
Complementary Events
Note! An important application is that
Example 1
You roll a die once. What’s the probability of getting two dots or more on the die?
Here you’re looking to find the probability of getting 2, 3, 4, 5 or 6 dots on the die. The easiest way to calculate this is to use the application above:
The event fewer than two dots is the same as getting one dot on the die, and the number of possibilities is then 1. That means you can write
The probability of getting at least two dots when you roll a die is then .
