This mouth we study Sequences and Series, I think is not very difficult. There have Arithmetic Sequences & Series and Geometric Sequences & Series. Arithmetic Sequences & Series has common difference but Geometric has common ratio.
Arithmetic Sequences & Series
Common Different: d=tn-t(n-1)
General Term: tn=a+(n-1)d
Ex. 3, 6, 9, …. 108 is which term?
108=3+(n-1)d
105=(n-1)3
35=n-1
n=36
108 is the 36th term.
Sum:
Geometric Sequences & Series have two way: Convergent and Divergent
Common Ratio: 
General Term: tn=ar^(n-1)
Ex. 3, 6, 12, 24, … find term 36th
t36=3(2)^36-1
t36=3(2)^35
t36=1.03*10^11
Sum:
