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Sum of an Arithmetic Sequence

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Calculate the value of the nth term of an arithmetic sequence and the sum from the first term to the nth term.
\(\normalsize Sn=a+(a+d)+(a+2d)+\cdots +(a+(n-1)d)\\\)
First term a
Common difference d
Number of terms n
n＝1,2,3 …
nth term an  
Sum Sn of an arithmetic sequence  
\(\normalsize Arithmetic\ progression\\ (1)\ a_n=a+(n-1)d\\ (2)\ Sn={\large\displaystyle \sum_{\small k=1}^{\small n}}a_k\\ \hspace{30px} =a+(a+d)+(a+2d)+\cdots +(a+(n-1)d)\\ \hspace{30px} ={\large\frac{n}{2}}(a+a_n)\\ \hspace{30px} ={\large\frac{n}{2}}(2a+(n-1)d)\\ \)

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Sum of an Arithmetic Sequence | Keisan