00:01
We're being asked to find the sum of this particular series.
00:04
So let's list the information that we know.
00:06
Well, we know the first term is 73, so a sub 1 is 73.
00:10
We know the last term in the series is 558, so a sub n is equal to 558.
00:17
And we can find our common difference, because to get from one term to the next, we simply just keep adding 5.
00:23
So d is equal to 5.
00:25
So we just need to find n.
00:27
To do this, we're going to use our formula to find the nth term in the arithmetic sequence, which remember is a sub n equals a sub 1 plus the quantity of n minus 1 times d.
00:38
So let's substitute in the values that we know.
00:41
So we're going to have 558 is equal to 73 plus the quantity of n minus 1 times 5.
00:50
So let's solve for n.
00:52
To do this, i'm going to first subtract 73 from both sides of our equation...