00:01
Okay, so we want to prove that a is a subset of b if and only if the complement of a union with b is equal to the universal set.
00:11
So we'll prove the left or right direction.
00:16
So i will say let a be a subset of b.
00:21
So this implies then, so if y is in the complement of b, this implies that y is not in b.
00:38
So because a is contained in b, this will imply that y is not in a as well.
00:48
So then this implies that y is not in a as well.
00:50
So then this implies that y is not in a.
00:53
In the a complement.
00:57
Now if you take this whole thing together you have that b bar is a subset subset of a bar.
01:07
Now what can we do with this? well we know that the universal set is equal to b bar union with b.
01:16
We also know that b is a subset of a so therefore this thing must be a subset of a union with or this would be yep.
01:29
But we know that this is also a subset of the universal set because every set inside the set is inside the universal set.
01:38
So what do we have? we have that the universal set is a subset of a union with b, which is also a subset of the universal set.
01:51
So therefore we have a union with b is equal.
01:58
Equal to your universal sense...