00:01
Okay, for every sequence that's arithmetic.
00:03
The nth term is given by this formula.
00:06
A1 is the first term, an n number of terms, and d the common difference.
00:13
So in the first case then, number one here, we got two, four, six.
00:18
So a1 is two, and d also is two.
00:23
The gap here is two.
00:26
So i want to work out, a15.
00:31
That will be a1 plus 14 times d.
00:36
Replace the n by 15.
00:39
So a1 is 2.
00:42
D also is 2.
00:45
We have 14, sorry, 2 plus 28.
00:49
So 30 is the first answer.
00:56
Now number 2, we have 13, 16, 19 and so on.
01:03
So the first term, a1, is 13.
01:07
The common difference, d, just do 16 minus 13 or 19 minus 16.
01:14
We get 3.
01:16
So i want to find the a25 here.
01:20
So a25 is a1, 13, n minus 1 is 24, d is 3.
01:29
So calculator 24 times 3 plus 13.
01:34
Is 85.
01:39
That's the answer here.
01:41
85.
01:44
Now the next one that we have is number three, 99, 88, 77, 66.
01:57
So the first term, a1 is 99.
02:01
Now we're going down in value, so d here must be negative...