1 Given that $$u _ { k } = \frac { 1 } { \sqrt { } ( 2 k - 1 ) } - \frac { 1 } { \sqrt { } ( 2 k + 1 ) }$$ express $\sum _ { k = 13 } ^ { n } u _ { k }$ in terms of $n$. Deduce the value of $\sum _ { k = 13 } ^ { \infty } u _ { k }$. \hfill \mbox{\textit{CAIE FP1 2014 Q1}}