00:01
I think one of my biggest suggestions with an arithmetic series, i know they call it a partial sum, but i call it a series, is to know the formula is the number of terms divided by two times the first term plus the last term.
00:19
So in this problem, you actually don't know how many terms you have, but if you read it closely, you know that the first term is 290.
00:32
And the last term is 119.
00:37
So if we look at what's going on, 290 plus, you actually only need to know those first two terms, 271 plus 252, is you can find the common difference from each of these.
00:52
So if you do 271, let me write d equals 271 minus 290, the common difference is negative 19, which you can confirm because if you can do minus 271 minus 19 again you get 252.
01:12
So what i'm trying to do is use the formula for an arithmetic sequence to the first term plus n minus 1 times the common difference.
01:23
I can figure out how many terms there are because the last term, whoops, forgot to put 119 is equal to the first term of 290.
01:35
Now we don't know what the common difference is, but we do, or sorry, we don't know what n is, but we know the common difference is negative 19.
01:43
So what i'm going to do is subtract 290 over...