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http://www.regentsprep.org/regents/math/algtrig/atp2/arithseq.htm
http://www.mathsisfun.com/algebra/sequences-sums-arithmetic.html
http://www.mathguide.com/lessons/SequenceArithmetic.html
http://www.purplemath.com/modules/series3.htm
http://www.mathwords.com/a/arithmetic_sequence.htm
http://www.ltcconline.net/greenl/courses/154/seqser/aritmet.htm
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