00:01
Since this is an arithmetic sequence, let me just write down the formula i like to use.
00:06
Something like this, a sub n equals a sub 1 plus n minus 1 times the common difference.
00:14
And so in the problem where they, i know they don't explicitly say it, but the first term is 1, and then the sixth term is negative 19.
00:25
And so what i can do is i can plug in 6 in for n, here and here.
00:30
Let me just write that real quick.
00:32
Which means that negative 19 is a sub 6.
00:38
And everything else, like negative, sorry, 1 is the first term, minus 1 times the common difference.
00:45
So i can figure out what the common difference is, because i can subtract 1 over.
00:50
That's negative 20.
00:51
6 minus 1 is 5d...