Problem 1
<h3>Answer: {1, 2, 3, 5, 7, 9, 11, 13}</h3>
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Explanation:
The notation A U B means we're applying a union between A and B. This will form a larger set in which items from A or B are thrown into A U B. Think of "union" as in getting married, and person A's stuff is combined with person B's stuff to form a larger set. Any duplicates are tossed. It's often helpful to sort from smallest to largest.
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Problem 2
<h3>Answer: 8</h3>
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Explanation:
Set C is defined as the set of prime numbers less than or equal to 20. The roster form is C = {2,3,5,7,11,13,17,19} which basically lists every item that fits the description earlier. Note how the value 1 is not prime, so it's not in the set C.
From here, we simply count out the values in set C to find there are 8 values. Therefore, n(C) = 8.
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Problem 3
<h3>Answer: 3, 4, 5, 6, 7</h3>
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Explanation:
Let's consider a real world example of set vs subset. Consider the set of all animals and consider the subset of all dogs. Let's say
A = all animals
B = all dogs
We can see that set A is clearly larger than B. Also, any member of set B is also in set A, but not the other way around. In other words, if something is a dog, then they are also an animal. However, this doesn't work the other way around. For instance, we could have a cat in set A that wouldn't be in set B.
Let's return to the problem at hand. Since B is a subset of A, this must mean everything in B = {2,12,x,9} is found in set A. Sure enough, the 2, 12 and 9 are all in set A. The x must also be a value in set A so we could have
x = 3
x = 4
x = 5
x = 6
x = 7
as our possible answers. I'm not listing x = 2, x = 9 or x = 12 as they have been taken care of. I'm assuming that your teacher wants {2,12,x,9} to list unique items (ie it's after the duplicates are tossed out).
