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Arithmetic series intro

Sal explains the formula for the sum of a finite arithmetic series. Created by Sal Khan.

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Video transcript

Let's say we have the simplest of arithmetic sequences and probably the simplest of sequences one, two... we're going to start at one and just increment by one one, two, three and we're going to go all the way to n And what I want to think about is what is the sum of this sequence going to be? And the sum of a sequence, we already know we call a series so what is the sum and I will just call it Sn what is this going to be equal to one plus two plus three plus going all the way to n well, we're going to do a neat little trick here where I'm going to rewrite this sum so I will write it again as Sn but now I'm just going to write it in reverse order I'm going to write it as n plus n minus one plus n minus two all the way to one and now I'm going to add these two equations So we know that Sn is equal to this so we are adding the same thing to both sides of this equation up here So on the left hand side we're going to have Sn plus Sn is just two times Sn and on the right hand side and this is where we start to see something kind of cool you have a one plus an n which is just going to be n plus one you have a two plus n minus one which is... well two plus n minus one is going to be n plus one again plus n plus one you have a three plus n minus two well that's going to be n plus one again I think you see what is going on here And we're going to go all the way to this last pair I guess you could say, or call it these last two terms and we have an n plus one again plus n plus one So how many of these n plus one's do we have? well we have n of them there were n of these terms in each of these equations one two three... all the way to n So, we can rewrite this thing as two times Sn is equal to, and you have n (n+1) terms so we could write it as n times n plus one n times n plus one and now to solve for Sn to solve for our sum we can just divide both sides by two And so we are going to be left with the sum from one to n this arithmetic sequence where we're just incrementing by one starting at one, is going to be equal to n times n plus one over two And this is neat because now you can quickly find the sum let's say from one to a hundred it will be 100 times 101 over 2 So very quickly you can find these sums And what I'm curious about and what we will explore in the next video is can we generalize this for any arithmetic sequence we started with a very simple one we started at one, we just incremented by one And it looks like so if I were to write it this way this is n times (n + 1) over 2 So this right over here, this n this is the nth term in our sequence and this right over here, this one was the first term in our sequence So at least in this case it looks like I took the average of the first term and the nth term so this right over here this is the average this right over here is the average of a1 and an and then I'm multiplying that times n and what I'm curious about is whether this is going to be true for any arithmetic sequence that the sum of it is going to be the average of the first and the last term times the number of terms