Posted by sonia on Monday, November 1, 2010 at 1:44pm.
First you have to find out how many terms there are, so find out which term number 84 is
84 = a + (n-1)d
84 = 12 + (n-1)(3)
72 = 3n - 3
3n = 75
n = 25
so now that we know we have 25 terms, just use the formula for the sum of 25 terms.
For more information in google type "wikipedia arithmetic progression"
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