27．如图，平行六面体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1的底面是菱形，且∠C1CB=∠C1CD=∠BCD=60∘\angle C_{1}CB=\angle C_{1}CD=\angle BCD=60\circ∠C1CB=∠C1CD=∠BCD=60∘，CD=CC1=2CD=CC_{1}=2CD=CC1=2，CD→=a→,CB→=b→,CC1→=c→\overrightarrow{CD}=\overrightarrow{a},\overrightarrow{CB}=\overrightarrow{b},\overrightarrow{C{C_1}}=\overrightarrow{c}CD=a,CB=b,CC1=c．（1）用空间的一个基底{a→,b→,c→}\left\{{\overrightarrow{a},\overrightarrow{b},\overrightarrow{c}}\right\}{a,b,c}表示AC1→\overrightarrow{A{C_1}}AC1，并求AC1AC_{1}AC1的长；（2）求异面直线CA1CA_{1}CA1与DC1DC_{1}DC1所成角的余弦值．
