[tex] \frac{ {y}^{2} - 2y + 1 }{1 - {y}^{2} } = \frac{(y - 1)}{(1 - y)(1 + y)} = \\ \frac{(y - 1)}{ - (y - 1)(1 + y)} = - \frac{1}{y + 1} [/tex]