Let $f ( x ) = x + \frac { a } { \pi ^ { 2 } - 4 } \sin x + \frac { b } { \pi ^ { 2 } - 4 } \cos x , x \in \mathbb { R }$ be a function which satisfies $f ( x ) = x + \int _ { 0 } ^ { \pi / 2 } \sin ( x + y ) f ( y ) d y$. Then $( a + b )$ is equal to\\
(1) $- \pi ( \pi + 2 )$\\
(2) $- 2 \pi ( \pi + 2 )$\\
(3) $- 2 \pi ( \pi - 2 )$\\
(4) $- \pi ( \pi - 2 )$