5. (a) Show that \(\frac { 2 } { n - 1 } - \frac { 2 } { n + 1 }\) can be expressed as \(\frac { 4 } { \left( n ^ { 2 } - 1 \right) }\).
(b) Hence, find an expression for \(\sum _ { r = 2 } ^ { n } \frac { 4 } { \left( r ^ { 2 } - 1 \right) }\) in the form \(\frac { ( a n + b ) ( n + c ) } { n ( n + 1 ) }\), where \(a , b , c\) are integers whose values are to be determined.
(c) Explain why \(\sum _ { r = 1 } ^ { 100 } \frac { 4 } { \left( r ^ { 2 } - 1 \right) }\) cannot be calculated.