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[itex]a_{n}[/itex] = ((last term) - (n-1)(common diff))

However, I think the person providing the youtube explanation makes a mistake in notation, which is causing me some confusion. If he has written this portion of his presentation correctly, then I will have to go back and review my confusion. So if anyone has a moment, could you fact check the portion of the derivation of this formula at exactly 9:16 - 9:23

http://www.youtube.com/watch?v=dbuwvw91fFc&list=PL56BFE1A5A41BC8D6

To me it seems [tex]a_{n}- a_{n-1} [/tex] should equal d, not -d , so did he intend to write [tex]a_{n} - a_{n+1} = -d[/tex]