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甲、乙两个工厂,甲厂位于一直线河岸的岸边\(A\)处,乙厂与甲厂在河的同侧,乙厂位于离河岸\(40\)千米的\(B\)处,乙厂到河岸的垂足\(D\)与\(A\)相距\(50\)千米,两厂要在此岸边\(AD\)之间合建一个供水站\(C\),从供水站到甲厂和乙厂的水管费用分别为每千米\(3a\)元和\(5a\)元,若\(CD=x\)千米,设总的水管费用为\(y\)元,如图所示,
\((\)Ⅰ\()\)写出\(y\)关于\(x\)的函数表达式;
\((\)Ⅱ\()\)问供水站\(C\)建在岸边何处才能使水管费用最省?