Let $f ( x ) = a ^ { x }$ $( a > 0 )$ be written as $f ( x ) = f _ { 1 } ( x ) + f _ { 2 } ( x )$, where $f _ { 1 } ( x )$ is an even function and $f _ { 2 } ( x )$ is an odd function. Then $f _ { 1 } ( x + y ) + f _ { 1 } ( x - y )$ equals:\\
(1) $2 f _ { 1 } ( x ) f _ { 1 } ( y )$\\
(2) $2 f _ { 1 } ( x + y ) f _ { 1 } ( x - y )$\\
(3) $2 f _ { 1 } ( x ) f _ { 2 } ( y )$\\
(4) $2 f _ { 1 } ( x + y ) f _ { 2 } ( x - y )$