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Kapag arithmetic sequence:
a_n=a_1+(n-1)d \\ S_n= \frac{n(a_1+a_n)}{2}  \\ n= \frac{a_n-a_1}{d} +1
Dito ang a subn ibig sabihin yun yung nth term, d yung common difference.

Kapag geometric sequence naman:
S_n= \frac{a_1(1-r^n)}{1-r}  \\ S_{\infty}= \frac{a}{1-r}
r yung common ratio