设\(S _{n}\)为首项不为零等差数列\(\{a _{n} \}\)的前\(n\)项和，已知\(a _{4} a _{5} =3a _{9}\)，\(S _{5} =20\)．
\((1)\)求数列\(\{a _{n} \}\)的通项公式；
\((2)\)设\(T _{n}\)为数列\(\{ \dfrac {1}{a_{n}a_{n+1}}\}\)的前\(n\)项和，求\( \dfrac {T_{n}}{a_{n+1}}\)的最大值．