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September 30, 2014

September 30, 2014

Posted by **Laura** on Saturday, June 16, 2007 at 8:17am.

If you mean the sum of the first 20 terms, beginning with k=1, of

ak = 3k - 5,

that would be -20 x 5 = -100

PLUS the sum of the first 20 terms of

ak = 3k

The latter is 3 times the sum of

1+2+3+...20 ,

or 3*(20)(21)/2= 630

So the answer is 630 - 100 = 530

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