8.
已知函数\(f(x)= \begin{cases} \overset{e^{-x}-2,(x\leqslant 0)}{2ax-1,(x > 0)}\end{cases}(a\)是常数且\(a > 0).\)对于下列命题:
\(①\)函数\(f(x)\)的最小值是\(-1\);
\(②\)函数\(f(x)\)在\(R\)上是单调函数;
\(③\)若\(f(x) > 0\)在\([ \dfrac {1}{2},+∞)\)上恒成立,则\(a\)的取值范围是\(a > 1\);
\(④\)对任意\(x_{1} < 0\),\(x_{2} < 0\)且\(x_{1}\neq x_{2}\),恒有\(f( \dfrac {x_{1}+x_{2}}{2}) < \dfrac {f(x_{1})+f(x_{2})}{2}\).
其中正确命题的序号是 ______ .