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