\[\begin{array}{l} M\left( {m;m + \sqrt {2m - {m^4}} } \right);N\left( {n;n + \sqrt {2n - {n^4}} } \right) \hfill \\ P\left( {p;p - \sqrt {2p - {p^4}} } \right);Q\left( {q;q - \sqrt {2q - {q^4}} } \right) \hfill \\ f\left( x \right) = x + \sqrt {2x - {x^4}} \hfill \\ g\left( x \right) = x - \sqrt {2x - {x^4}} \hfill \\ \left\{ \begin{array}{l} f'\left( m \right) = g'\left( p \right) \hfill \\ f'\left( n \right) = g'\left( q \right) \hfill \\ f'\left( n \right) \cdot f'\left( m \right) = - 1 \hfill \\ \left| {AB} \right| = \left| {AD} \right| \hfill \\ \end{array} \right. \hfill \\ \end{array}\]