已知函数\(f(x)=\dfrac{1}{3}{{x}^{3}}+a{{x}^{2}}+bx\),且\(f{{'}}(-1)=0\)
\((1)\)试用含\(a\)的代数式表示\(b\);
\((2)\)求\(f(x)\)的单调区间;
\((3)\)当\(a=-1\)时,\(f(x)\)在\(x_{1}\),\(x_{2}(x_{1} < x_{2})\)处取得极值,记点\(M(x_{1},f(x_{1}))\),\(N(x_{2},f(x_{2}))\)证明:线段\(MN\)与曲线\(f(x)\)存在异于\(M\)、\(N\)的公共点.