1965 AHSME Problems/Problem 20
In order to calculate the 
th term of this arithmetic sequence, we can subtract the sum of the first 
terms from the sum of the first terms of the sequence. Plugging in and as values of 
in the given expression and subtracting yields ![\[(3r^2+2r)-(3r^2-4r+1)\]](http://latex.artofproblemsolving.com/9/1/f/91fe4d38f54782c52ef2b84db70dcfd7cf64ba6c.png)
. Simplifying gives us the final answer of 
.
