\((1)\)选修\(4-4\):坐标系与参数方程
在直角坐标系\(xoy\)中,以坐标原点为极点,\(x\)轴正半轴为极轴建立极坐标系,半圆\(C\)的极坐标方程为\(\rho =2\cos \theta \),\(\theta \in \left[ 0,\dfrac{\pi }{2} \right]\) .
\((\)Ⅰ\()\)求\(C\)的参数方程;
\((\)Ⅱ\()\)设点\(D\)在\(C\)上,\(C\)在\(D\)处的切线与直线\(y= \sqrt{3}x+2 \)垂直,根据\((\)Ⅰ\()\)中你得到的参数方程,确定\(D\)的坐标.
\((2)\)选修\(4-5\):不等式选讲
设函数\(f\left(x\right)=\left|x+ \dfrac{1}{a}\right|+\left|x-a\right|\left(a > 0\right) \)
\((\)Ⅰ\()\)证明:\(f\left(x\right)\geqslant 2 \) ;
\((\)Ⅱ\()\)若\(f\left(3\right) < 5 \),求\(a \)的取值范围.