Given that \(n\) is a positive integer, express
$$\frac { 7 } { 3 + 5 \sqrt { n } } - \frac { 7 } { 5 \sqrt { n } - 3 }$$
as a single fraction not involving surds.
7
Hence, deduce that
$$\frac { 7 } { 3 + 5 \sqrt { n } } - \frac { 7 } { 5 \sqrt { n } - 3 }$$
is a rational number for all positive integer values of \(n\)