如图,在长方体\(ABCD-{{A}_{1}}{{B}_{1}}{{C}_{1}}{{D}_{1}}\),中,\(AD=A{{A}_{1}}=1,AB=2\),点\(E\)在棱\(AD\)上移动.
![](https://www.ebk.net.cn/tikuimages/2/2017/600/shoutiniao93/d31ae4c4cde9baa8c1df45d9ef3eba22.png)
\((1)\)证明:\({{D}_{1}}E\bot {{A}_{1}}D\);
\((2)\)当\(E\)为\(AB\)的中点时,求点\(E\)到面\(AC{{D}_{1}}\)的距离;
\((3)AE\)等于何值时,二面角\({{D}_{1}}-EC-D\)的大小为\(\dfrac{\pi }{4}\).