00:01
So in this problem, we're given the arithmetic sequence 4, 16, 18, 20, etc.
00:06
And we want to find the sum of the first 120 terms.
00:09
Well, to do that, we can use our formula, s -sub -n, is equal to n -d -by -2 times the quantity of 2 times a -1, plus n -minus 1 times d.
00:21
So let's think about what we know in this particular sequence.
00:24
Well, our first term is 14, so a -1 is 14.
00:28
We know that there's 120 terms, so n is equal to 120, and we can find d because to get from one term to the next, we keep adding two.
00:38
So d is equal to two.
00:41
So let's substitute our values into this formula...