\[\begin {array} {1 1} (1)\;\frac{a^2b^2}{a^2-b^2} & \quad (2)\;\frac{a^2b^2}{a^2+b^2} \\ (3)\;\frac{a^2+b^2}{a^2b^2} & \quad (4)\;\frac{a^2-b^2}{a^2b^2} \end {array}\]