3.
已知椭圆\(C\):\(\dfrac{{{x}^{2}}}{{{a}^{2}}}+\dfrac{{{y}^{2}}}{{{b}^{2}}}=1\left( a > b > 0 \right)\)的左、右焦点分别为\({F}_{1}\left(-c,0\right) \)和\({F}_{2}\left(c,0\right) \),椭圆交\(y\)轴于\(S\),\({{S}_{\vartriangle OS{{F}_{2}}}}=\dfrac{\sqrt{3}}{2}\),离心率\(e < \dfrac{ \sqrt{3}}{2} \),直线\(l\)过点\(P\left(0,-c\right) \)交椭圆于\(A\), \(B\)两点,当直线\(l\)过点\({F}_{2} \)时,\(\vartriangle {{F}_{1}}AB\)的周长为\(8\).
\((1)\)求椭圆\(C\)的标准方程;
\((2)\)对于椭圆\(\dfrac{{{x}^{2}}}{{{a}^{2}}}+\dfrac{{{y}^{2}}}{{{b}^{2}}}=1\left( a > b > 0 \right)\)的切线有如下性质:若点\(\left( {{x}_{0}},{{y}_{0}} \right)\)是椭圆上的点,则椭圆在该点处的切线方程为\(\dfrac{{{x}_{0}}x}{{{a}^{2}}}+\dfrac{{{y}_{0}}y}{{{b}^{2}}}=1.\)若动点\(P\)在直线\(x+y=3 \)上,经过点\(P\)的直线\(m\),\(n\)与椭圆\(C\)相切,切点分别为\(M\),\(N.\)求证:直线\(MN\)必经过一定点.