9.
如图所示,直三棱柱\(ABC—A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\),底面\(\triangle ABC\)中,\(CA=CB=1\),\(∠BCA=90^{\circ}\),棱\(AA\)\({\,\!}_{1}\)\(=2\),\(M\),\(N\)分别是\(A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\),\(A\)\({\,\!}_{1}\)\(A\)的中点.
![](https://www.ebk.net.cn/tikuimages/2/2017/600/shoutiniao68/4392eb011ead097615410b94339a2d4f.png)
\((\)Ⅰ\()\)求\(\overrightarrow{BN}\) 的模;
\((\)Ⅱ\()\)求\(\cos 〈\overrightarrow{BA_{1}}\),\(\overrightarrow{CB_{1}}〉\)的值;
\((\)Ⅲ\()\)求证:\(A_{1}B⊥C_{1}M.\)