Recall the method of partial fraction decomposition: try to find A , B {\displaystyle A,B} so that 1 ( x + 1 ) ( x − 3 ) = A x + 1 + B x − 3 . {\displaystyle {\frac {1}{(x+1)(x-3)}}={\frac {A}{x+1}}+{\frac {B}{x-3}}.}
