A Union B Formula
The union of two sets A and B is a set that contains all the elements of A and B and is denoted by A U B (which can be read as "A or B" (or) "A union B"). A union B formula is used to find the union of two sets A and B. The union can be found by just putting all the elements of A and B in one set and removing duplicates. i.e., A U B can be found without using the A union B formula also.
Let us learn about the A union B formula with a few examples in the end.
1.  What Is the A Union B Formula? 
2.  Number of Elements in A union B Formula 
3.  What is A union B Probability Formula? 
4.  FAQs on A Union B Formula 
What Is the A Union B Formula?
The union of two sets is represented by writing the symbol "U" in between the two sets. The formula for A union B means that any element that is present either in A or in B is present in A U B. By using the above definition, the A union B formula is,
A U B = {x : x ∈ A (or) x ∈ B}
Number of Elements in A union B Formula
Suppose we have two sets A and B. Now, A U B consists of elements of both sets A and B, taken one at a time (leaving duplicates). We have the following representations:
 n(A U B) = Number of elements in A U B
 n(A) = Number of elements in A
 n(B) = Number of elements in B
 n(A ∩ B) = Number of elements that are common to both A and B
n(A) + n(B) gives the total number of elements that are in A and the elements in B including the elements that are common. This implies that the number of common elements is counted twice. Hence, to balance that and make sure that all the elements are counted just once, we subtract n(A ∩ B) from n(A) + n(B) and hence the formula for the number of elements in A U B is:
n(A U B) = n(A) + n(B)  n(A ∩ B)
What is A union B Probability Formula?
Using the same logic as above and to avoid counting the same elements twice, we have the formula for P(A U B) (the probability formula of A union B) as:
P(A ∪ B) = P(A) + P(B)  P(A ∩ B)
If A and B are mutually exclusive, then we have P(A ∪ B) = P(A) + P(B) as P(A ∩ B) = 0.
Important Notes on A union B Formula:
 A U B = {x : x ∈ A (or) x ∈ B}
 n(A U B) = n(A) + n(B)  n(A ∩ B)
 P(A ∪ B) = P(A) + P(B)  P(A ∩ B)
 A U B = B U A
☛ Related Topics:
A Union B Formula Examples

Example 1: If A = {1, 2, 3, 4} and B = {2, 3, 6, 7}, then find A U B.
Solution: Given that:
A = {1, 2, 3, 4}
B = {2, 3, 6, 7}
By using the A union B formula, we find A U B just by writing all the elements of A and B in one set by avoiding duplicates.
So, A U B = {1, 2, 3, 4} U {2, 3, 6, 7} = {1, 2, 3, 4, 6, 7}
Note: The elements in the answer set do not need to be in order.
Answer: A U B = {1, 2, 3, 4, 6, 7}

Example 2: Find A U B using the following figure.
Solution: By using the A union B formula, we find A U B just by writing all the elements of A and B in one set by avoiding duplicates.
Thus, by the given Venn Diagram, A U B = {11, 20, 14, 2, 10, 15, 30}.
Note: 17, 16, 3, and 18 are neither the elements of A nor the elements of B and hence they are not present in A U B.
Answer: A U B = {11, 20, 14, 2, 10, 15, 30}.

Example 3: Determine the number of elements in A union B if n(A) = 15, n(B) = 7 and n(A ∩ B) = 3.
Solution: To determine the number of elements in A U B, we will use the formula n(A U B) = n(A) + n(B)  n(A ∩ B).
n(A U B) = 15 + 7  3
= 22  3
= 19
Answer: Hence, the number of elements in A union B is 19.

Example 4: Determine the probability of randomly getting an ace or a black card from a deck of 52 playing cards.
Solution: We know that there are 26 red cards and 26 black cards in a deck of 52 playing cards and four aces in total out of which 2 are red and 2 are black.
Let A be the event of getting an ace and B be the event of getting a black card. Then, A ∩ B is the event of getting a black ace card.
P(A) = 4/52 = 1/13
P(B) = 26/52 = 1/2
P(A ∩ B) = 2/52 = 1/26
So, the probability of getting an ace or a black card is P(A U B) which is given by,
P(A) + P(B)  P(A ∩ B)
= 4/52 + 26/52  2/52
= 28/52
= 7/13
Answer: Hence the required probability is 7/13.
FAQs on A Union B Formula
What is A union B Formula in Math?
By using the definition of A U B, the A union B formula is, A U B = {x : x ∈ A (or) x ∈ B} which implies A U B consists of elements that are either in A or in B.
How Do You Calculate AUB?
A U B can be calculated by taking the elements of A and the elements of B and considering the elements that are common in both A and B only once and combining them in one set.
What is the Union of A and B?
The union of two sets A and B is a set that contains all the elements of A and B taken one at a time and is denoted by A U B.
What is A union B Whole Complement Formula?
The formula for A union B whole complement formula is given by, (AUB)^{c} = A^{c} ∩ B^{c}. It is also called the DeMorgan's Law of Union of Sets.
How to Find the Number of Elements in A union B?
The number of elements in A union B can be calculated by counting the elements in A and B and taking the elements that are common only once. The formula for the number of elements in A union B is n(A U B) = n(A) + n(B)  n(A ∩ B).
visual curriculum