00:01
The series of this month stephen is 200, 200, 200, these are the first 3 months, and then, 240, 280, and etc.
00:20
And we assume there are n terms, and for the n's term, it's 200 plus 40 times n times n minus 3.
00:36
You can check this when n equals 3 is 200, then n equals 4 is 240 and etc.
00:48
And so the sum of all these n terms equals 200 plus 200, and then for the following terms, they are an arithmetic series, and hence we can use the formula of the sum, it equals the first term, plus the lesser term and then times the number of the terms there are n minus 2 terms and then divided by 2 so this equals 400 plus m minus 2 over 2 times there's a typo here 4hander plus 40 n minus 120 and this equals 11 ,040.
02:05
We can simplify this expression, so it becomes 20 squared plus a hundred and minus 280 equals 1 ,000, sorry, 10 ,000 and 644.
02:31
And 640...