21. Given the function $f ( x ) = \frac { a x } { ( x + r ) ^ { 2 } } ( a > 0 , r > 0 )$\\
(1) Find the domain of $f ( x )$ and discuss the monotonicity of $f ( x )$;\\
(2) If $\frac { a } { r } = 400$, find the extreme values of $f ( x )$ on $( 0 , + \infty )$.