如图\(1\),在\(\triangle ABC\)中,\(| \overrightarrow{AB}|=2\),\(| \overrightarrow{AC}|=1\),点\(D\)是\(BC\)的中点.
\((I)\)求证:\( \overrightarrow{AD}= \dfrac { \overrightarrow{AB}+ \overrightarrow{AC}}{2}\);
\((II)\)直线\(l\)过点\(D\)且垂直于\(BC\),\(E\)为\(l\)上任意一点,求证:\( \overrightarrow{AE}\cdot ( \overrightarrow{AB}- \overrightarrow{AC})\)为常数,并求该常数;
\((III)\)如图\(2\),若\(\cos = \dfrac {3}{4}\),\(F\)为线段\(AD\)上的任意一点,求\( \overrightarrow{AF}\cdot ( \overrightarrow{FB}+ \overrightarrow{FC})\)的范围.