Let $$I = \int\limits_0^I {{{{x^3}\cos 3x} \over {2 + {x^2}}}dx} $$, then

Let f be a non-constant continuous function for all x $$ \ge $$ 0. Let f satisfy the relation f(x) f(a $$-$$ x) = 1 for some a $$ \in $$ R⁺. Then, $$I = \int_0^a {{{dx} \over {1 + f(x)}}} $$ is equal to