Gie it h \(\mathrm { f } ( r ) = \frac { 1 } { ( r + 1 ) ( r + 2 ) } , \mathrm { s }\) th t
$$\mathrm { f } \left( r - 1 \quad \mathrm { f } ( r ) = \frac { 2 } { r ( r + 1 ) ( r + 2 ) } \right.$$
Hen e fid \(\sum _ { r = 1 } ^ { n } \frac { 1 } { r ( r + 1 ) ( r + 2 ) }\).
Ded e th le \(6 \sum _ { r = 1 } ^ { \infty } \frac { 1 } { r ( r + 1 ) ( r + 2 ) }\).