00:01
Hey folks, so we are going to use induction to prove that a set of cardinality n has n times n minus 1 over 2 distinct two element subsets.
00:12
And i'll go ahead and say n is greater than are equal to 2.
00:15
This formula does work when n is equal to 1.
00:17
You get 0.
00:19
But i went ahead and started kind of with the relevant number, right? one thing i want to point out quickly to is in the question that i am answering, in the original question, this here had a plus sign here, and that is not the correct formula.
00:36
So i went ahead and replaced it with the correct one, and we'll go ahead and answer the question with this slight change.
00:42
Okay, so the first thing we need to do is check the base case.
00:50
So as i said, when you plug in 1, you get 0, which is correct for a 1 element set.
00:54
Let's go ahead and check when n is equal to 2, and this is actually relevant.
00:57
So we plug in 2, and this is indeed 1.
01:05
And this is a good thing because a set with two elements has exactly one two elements subset, right? so great.
01:15
So now we're going to go ahead and assume the formula holds for an arbitrary n.
01:37
And then we're going to prove that it also holds for n plus 1.
01:42
So we want to show the formula also...