3.
已知函数\(f(x)=\dfrac{mx-n}{x}-\ln x,(m,n\in R)\)
\((1)\)若函数\(f(x)\)在\(\left( 2,f(2) \right)\)处的切线与直线\(x-y=0\)平行,求实数\(n\)的值
\((2)\)讨论函数\(f(x)\)在区间\(\left[ 1,+\infty \right)\)上的最大值;
\((3)\)若\(n=1\)时,函数\(f(x)\)恰有两个零点\({{x}_{1}},{{x}_{2}}(0 < {{x}_{1}} < {{x}_{2}})\),求证:\({{x}_{1}}+{{x}_{2}} > 2\)