00:01
Okay, so we're given that a set of numbers is considered closed under an operation.
00:07
If applying the operation to any two numbers in the set results in a number, another number in the sets.
00:14
So we want to determine if our following operations and sets is closed.
00:21
So here we have our positive integers and subtraction.
00:25
So let's take two positive integers.
00:28
So let's choose a is equal to 2 and b is equal to 3.
00:35
And now if we subtract these two, if we take a minus b, this should be a positive integer if it's closed.
00:42
But we get 2 minus 3, which is equal to negative 1.
00:45
So this is a negative integer.
00:47
So we see that this sets and operation is not closed.
00:56
Okay, and for part b, we have our integer and addition and subtraction.
01:01
So let's note that if we have any integer and we subtract them, we get another integer in our set that is 8 minus b...