已知角\(α\)的顶点与原点\(O\)重合，始边与\(x\)轴的非负半轴重合，它的终边与单位圆交点为\(P(- \dfrac {4}{5}, \dfrac {3}{5})\)．
\((\)Ⅰ\()\)求\(\cos (α+ \dfrac {π}{4})\)和\(\sin 2α\)的值；
\((\)Ⅱ\()\)求\( \dfrac {3\sin α-2\cos α}{5\cos \alpha +3\sin \alpha }\)的值．