Here is a great website to understand sequences and series and the formulas involved for geometric and arithmetic sequences.

www.coolmath.com/algebra/19-sequences-series/01-whats-a-sequence-05

Here's another:

http://www.mathsisfun.com/algebra/sequences-series.html

Practice problems

www.ixl.com/math/algebra-1/arithmetic-sequences

www.ck12.org/calculus/sequences/

sequences.docx

mathbitsnotebook.com/Algebra1/Functions/FNSequencesPractice.html

www.coolmath.com/algebra/19-sequences-series/01-whats-a-sequence-05

Here's another:

http://www.mathsisfun.com/algebra/sequences-series.html

Practice problems

www.ixl.com/math/algebra-1/arithmetic-sequences

www.ck12.org/calculus/sequences/

sequences.docx

mathbitsnotebook.com/Algebra1/Functions/FNSequencesPractice.html