The difference between the maximum and minimum values of the function $f ( x ) = a ^ { \cos x }$, where $a > 0$ and $x$ is real, is 3 . Find the sum of the possible values of $a$.
..... 17
The difference between the maximum and minimum values of the function $f ( x ) = a ^ { \cos x }$, where $a > 0$ and $x$ is real, is 3 .
Find the sum of the possible values of $a$.