soluzione Risolvere la seguente equazione x x2 - 9   =   1 x + 1 x(x + 1) (x + 1)(x - 3)(x + 3)   =   x2 - 9 (x + 1)(x - 3)(x + 3) x + 1 ≠ 0 → x ≠ -1 x - 3 ≠ 0 → x ≠ + 3 x + 3 ≠ 0 → x ≠ -3 C.R. →   x ≠ -1;   x ≠ ± 3 x(x + 1) (x + 1)(x - 3)(x + 3) ·(x + 1)(x - 3)(x + 3)   =   x2 - 9 (x + 1)(x - 3)(x + 3) ·(x + 1)(x - 3)(x + 3)    x(x + 1) = x2 - 9    x2 + x = x2 - 9    x2 - x2 + x = - 9    x = - 9