Links
[1] https://hootsgo.org/?q=node/6732
[2] http://hotmath.com/help/gt/genericalg2/section_9_2.html
[3] http://www.analyzemath.com/math_problems/arith-seq-problems.html
[4] http://www.regentsprep.org/regents/math/algtrig/atp2/sequencepractice.htm