$2.($a$)$ Find the formula for $s_n,$ the $n^{th}$ partial sum of the series ${\displaystyle \underset{k=1}{\overset{}{}}}(\frac{1}{k^3}-\frac{1}{(k+1{)}^3}).($b$)$ Compute ${\displaystyle \underset{k=1}{\overset{}{}}}(\frac{1}{k^3}-\frac{1}{(k+1{)}^3}).($Hint: Use the formula from part $($a$).)$