\([[\)选修\(4―4\):坐标系与参数方程\(]\)
在直角坐标系\(xOy\)中,直线\(l\)\({\,\!}_{1}\)的参数方程为\(\begin{cases}x=2+t \\ y=kt\end{cases} (\)\(t\)为参数\()\),直线\(l\)\({\,\!}_{2}\)的参数方程为\(\begin{cases}x=-2+m \\ y= \dfrac{m}{k}\end{cases} (\)\(m\)为参数\().\)设\(l\)\({\,\!}_{1}\)与\(l\)\({\,\!}_{2}\)的交点为\(P\),当\(k\)变化时,\(P\)的轨迹为曲线\(C\).
\((1)\)写出\(C\)的普通方程;
\((2)\)以坐标原点为极点,\(x\)轴正半轴为极轴建立极坐标系,设\(l\)\({\,\!}_{3}\):\(ρ\)\((\cos \)\(θ\)\(+\sin \)\(θ\)\()− \sqrt{2} =0\),\(M\)为\(l\)\({\,\!}_{3}\)与\(C\)的交点,求\(M\)的极径.