00:01
What i did was the arithmetic series formula.
00:03
I provided the formula.
00:05
So n is the number of terms being added.
00:09
So that's number of terms being added of terms being added.
00:19
And a1 is the first term.
00:22
So 3n is the first term.
00:25
And then nth is the last term.
00:30
Okay.
00:30
So this is the last term.
00:31
And this is the first term, okay? and so, and then you divide by two.
00:39
So how they got this is, n, we don't know the number of terms being added, okay? then i'm going to multiply it by a1, which is the first term, so 1 plus 3n minus 2, and that's divided by 2...