如图,在\(\triangle ABC\)中,点\(N\)为\(AC\)边的任意一点,\(D\)为线段\(AB\)上一点,若\(∠MPN\)的顶点\(P\)为线段\(CD\)上一点,其两边分别与边\(BC\),\(AC\)交于点\(M\)、\(N\),且\(∠MPN+∠ACB=180^{\circ}\).
\((1)\)如图\(1\),若\(AC=BC\),\(∠ACB=90^{\circ}\),且\(D\)为\(AB\)的中点时,则\(\dfrac{PM}{PN}=\)________\(.\)请证明你的结论;
\((2)\)如图\(2\),若\(BC=m\),\(AC=n\),\(∠ACB=90^{\circ}\),且\(D\)为\(AB\)的中点时,则\(\dfrac{PM}{PN}=\)________\(.\)请证明你的结论;
\((3)\)如图\(3\),若\(\dfrac{BD}{AB}=k\),\(BC=m\),\(AC=n\),请直接写出\(\dfrac{PM}{PN}\)的值\(.(\)用\(k\),\(m\),\(n\)表示\()\)