9.
\((1)\)已知\(α\)是第二象限的角,\(\tan α=\dfrac{1}{2}\),则\(\cos α=\)________.
\((2)\)已知\(a=x^{α}\),\(b={x}^{ \frac{α}{2}} \),\(c={x}^{ \frac{1}{α}} \),\(x∈(0,1)\),\(α∈(0,1)\),则\(a\),\(b\),\(c\)的大小关系是________\( (\)用“\( < \)”连接\()\).
\((3)\)在\(\triangle ABC\)中,\(D\)为\(BC\)边上一点,\(BC=3BD\),\(AD=\sqrt{2}\),\(∠ADB=135^{\circ}.\)若\(AC=\sqrt{2}AB\), 则\(BD=\)________.
\((4)\)给出定义:若函数\(f(x)\)在\(D\)上可导,即\(f′(x)\)存在,且导函数\(f′(x)\)在\(D\)上也可导,则称\(f(x)\)在\(D\)上存在二阶导函数,记\(f″ (x)=(f′(x))′.\)若\(f″(x) < 0\)在\(D\)上恒成立,则称\(f(x)\)在\(D\)上为凸函数\(.\)以下四个函数在\(\left(\begin{matrix}0, \dfrac{π}{2} \end{matrix}\right)\)上是凸函数的是________\((\)把你认为正确的序号都填上\()\).
\(①f(x)=\sin x+\cos x\);\(②f(x)=\ln x-2x\);\(③f(x)=-x^{3}+2x-1\);\(④f(x)=xe^{x}\).