In this explainer, we will learn how to find the probability of the difference of two events.
First, recall the operations on events that you have met so far.
Definition: Complement, Intersection, and Union of Events
The operations on events and are as follows, with the shaded area in the Venn diagram representing each operation respectively.
- The complement of an event is denoted by and contains elements that are not in .
- The intersection of events and is denoted by and contains elements that are in both and .
- The union of events and is denoted by and contains elements that are in either or or both.
The new operation that we will meet in this explainer is the difference between two events and , as detailed in the definition below.
Definition: Difference of Events
The difference between two events and is denoted by and is illustrated by the shaded area in the Venn diagram below. This contains elements that are in but not in .
Using our understanding of Venn diagrams, we can derive the formula for the difference between two events.
By considering the area of the shaded region for , this is equivalent to the area of minus the area of , as seen below.
Therefore, . We can then use this to derive a formula for the probability of the difference between two events.
Rule for the Probability of the Difference between Two Events
The probability of the difference between two events and is
In the following example, we will apply the rule of probability in the definition above in order to find the probability of the difference between two events.
Example 1: Determining the Probability of the Difference between Two Events
Suppose and are two events. Given that and , determine .
Answer
Recall that the formula for the probability of the difference between two events and is
By substituting and into the formula above, we get
Therefore, .
In the next example, we will apply the rule of probability for the difference between two events in order to determine the probability of an event, where the difference and intersection are known.
Example 2: Determining the Probability of an Event given the Difference and Intersection of Two Events
Suppose that and are two events. Given that and , determine .
Answer
The formula for the probability of the difference between two events and is
By substituting and , we get the following equation:
If we rearrange to make the subject, we get
Therefore, .
In the next example, we will consider how to find the probability of the difference between two events given in a context.
Example 3: Finding the Probability of the Difference between Two Events Given in a Context
A ball is drawn at random from a bag containing 12 balls each with a unique number from 1 to 12. Suppose is the event of drawing an odd number and is the event of drawing a prime number. Find .
Answer
To find , we use the formula for the probability of the difference between two events and , which is
In order to do this, we must find and .
To find , we first identify the set . We know that is the event of drawing an odd number from a bag with balls numbered from 1 to 12. Therefore, is given by the set .
Since the number of outcomes in is 6 and the total number of outcomes is 12 (since there are 12 balls in the bag), therefore the probability of is given by
In order to find , we start by identifying the sets and , and the set . We know that is given by the set (as stated above). Set is the event of drawing a prime number from a bag with balls numbered from 1 to 12. Therefore, is given by the set .
We can see that , the intersection of and , is the set that contains the elements that occur in both and . In this case, . The probability of is given by
We can now substitute and into the formula in order to find :
Therefore, .
Next, we will consider the probability of the difference between two events, where one event is a subset of another.
Recall that if is a subset of , then all of the elements in are in and that the intersection of and is , or . This can be seen in the Venn diagram below:
We can then use this to find the rules of probability for the difference between two events.
Definition: Probability of the Difference between Two Events, Where One Is a Subset of Another
For two events and , where is a subset of , denoted , it follows that the probability of the difference between two events is as follows:
- , since ,
- , since .
In the next example, we will use the rules for the probability of the difference between two events when one is a subset of another event.
Example 4: Determining the Probability of an Event given the Difference between Two Events, Where One Is a Subset
Suppose and are two events. Given that , , and , determine .
Answer
For two events and , where , we know that . For the difference of two events, this gives us
Since and , by substitution we can form the equation
Therefore, rearranging to make the subject gives us
Therefore, .
We can use multiple rules of probability for operations on events in order to solve problems. We will next consider two of these rules of probability, the complement and the union of events. Let’s recall what these rules are.
Definition: Rules of Probability for Complement and Union of Events
- The probability of the complement of an event is
- The probability of the union of events and is
The following example uses the rules of probability for the union of two events and the difference between two events.
Example 5: Determining the Probability of the Difference of Two Events Using the Addition Rule
Suppose and are two events with probabilities and . Given that , determine .
Answer
As we are required to find , we must use the rule of probability for the difference between two events, which is stated as follows:
Since we do not know , we must use a further rule to determine this. As we know as well as and , we can use the addition rule for probability:
When we substitute , , and , we get an equation with :
Rearranging to solve for gives us
Since we have found , we can substitute this, along with , into the formula
So, by substituting, we can solve for :
Therefore, .
The next example uses the rules of probability for the complement of an event, the union of an event, and the difference between two events.
Example 6: Determining the Probability of the Difference of Two Events Using the Addition Rule and the Complement Rule
Suppose that and are events in a random experiment. Given that , , and , determine .
Answer
As we are required to find , we must use the rule of probability for the difference between two events, which is stated as follows:
As events and are reversed in this formula, we need to rewrite this as since .
Since we know , , and , but not or , we need to use rules of probability for the complement of an event and the probability of the union of two events. First, we will use the rule of probability for a complement of an event to find .
We know that
So, to find , we substitute and rearrange for :
Having found , we can use the addition formula to find . This is
By substituting , , and and then rearranging to make the subject, we get
Since we have found , we can now use this, along with , to find . We do this by substituting into the formula and solving for :
Therefore, .
In this explainer, we have learned about the probability rule for the difference between two events and . We have seen this applied to examples where only this rule is applied or where further rules, such as those for the complement of a set or the union of two sets, are used.
Key Points
- The difference between two sets and is denoted by and is represented on the Venn diagram below:
- The rule of probability for the difference between two events and is
- The rules of probability for the difference between two events, when is a subset of , are
- ,
- .
