\(21.\)已知\(F_{1}\),\(F_{2}\)是椭圆\( \dfrac{x^{2}}{a^{2}}+ \dfrac{y^{2}}{b^{2}}=1(a > b > 0)\)的两个焦点,离心率为\( \dfrac{1}{2}\),\(P\)为椭圆上的一点,且\(∠F_{1}PF_{2}=60^{\circ}\),\(\triangle PF_{1}F_{2}\)的面积为\( \sqrt{3}\).
\((1)\)求椭圆的方程;
\((2)\)若直线\(l\):\(y=- \dfrac{1}{2}x+m\)与椭圆交于\(A\),\(B\)两点,与以\(F_{1}F_{2}\)为直径的圆交于\(C\),\(D\)两点,且满足\( \dfrac{|AB|}{|CD|}= \dfrac{5 \sqrt{3}}{4}\),求直线\(l\)的方程.