If the curves, $\frac { x ^ { 2 } } { a } + \frac { y ^ { 2 } } { b } = 1$ and $\frac { x ^ { 2 } } { c } + \frac { y ^ { 2 } } { d } = 1$ intersect each other at an angle of $90 ^ { \circ }$, then which of the following relations is TRUE?\\
(1) $a - c = b + d$\\
(2) $a - b = c - d$\\
(3) $a + b = c + d$\\
(4) $a b = \frac { c + d } { a + b }$