Calculating the $100th$ term of a series I came across a series
$1,5,13,25,41,61.......$
I have to calculate the $100^{th}$ term of this series. How do I do it?
Is there any formula to calculate the $n^{th}$ term of a series?

In general, there is no way to determine the $n$th term of a series.
For this series 1, 5, 13, 25, 41, 61, ...
$1 + 4\times 1 = 5$ 
$5 + 4\times 2 = 13$
$13 + 4\times 3 = 25$
...
$\Rightarrow f(n)=f(n-1)+4(n-1), f(0)=1 \Rightarrow f(n)=(4\sum_{i=1}^{n-1}i)+1 $
$f(100)=(4\sum_{i=1}^{99}i)+1=19801$
