已知斜率为\(k\)的直线\(l\)与椭圆\(C\):\( \dfrac {x^{2}}{4}+ \dfrac {y^{2}}{3}=1\)交于\(A\),\(B\)两点,线段\(AB\)的中点为\(M(1,m)(m > 0)\).
\((1)\)证明:\(k < - \dfrac {1}{2}\);
\((2)\)设\(F\)为\(C\)的右焦点,\(P\)为\(C\)上一点,且\( \overrightarrow{FP}+ \overrightarrow{FA}+ \overrightarrow{FB}= \overrightarrow{0}.\)证明:\(| \overrightarrow{FA}|\),\(| \overrightarrow{FP}|\),\(| \overrightarrow{FB}|\)成等差数列,并求该数列的公差.