If the $n$th partial sum of a series ${\displaystyle \underset{n=1}{\overset{}{}}}{a}_n$ is find $a_n$ and ${\displaystyle \underset{n=1}{\overset{}{}}}{a}_n.a_1=a_n=(forn1){\displaystyle \underset{n=1}{\overset{}{}}}{a}_n=$