00:02
We're asked to find a1, given that a6 is equal to 12, and a14 is equal to 28.
00:10
Well, based on our formula, we know that a6, or an is equal to a1 plus d and minus 1.
00:20
So that's 12 is equal to a1 plus d, 12 minus 1.
00:27
That gives me a1 plus 11d, and 28 is equal to a1 plus d, d.
00:34
28 minus 1.
00:36
That gives me a1 is plus 27d.
00:41
Okay.
00:44
So let's write that here.
00:45
Let's multiply this by negative.
00:48
Actually, we get negative 12 is equal to negative a1 minus 11d.
00:55
So that gives me 28 minus 12, which is 16 is equal to our a1s cancel, 27 minus 11, 16...