10.
.如图,在棱长为\(a\)的正方体\(ABCD-A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\)\(D\)\({\,\!}_{1}\)中,点\(E\)是棱\(D\)\({\,\!}_{1}\)\(D\)的中点,点\(F\)在棱\(B\)\({\,\!}_{1}\)\(B\)上,且满足\(B\)\({\,\!}_{1}\)\(F=\)\(2\)\(BF\).
\((1)\)求证:\(EF\)\(⊥\)\(A\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1};\)
\((2)\)在棱\(C\)\({\,\!}_{1}\)\(C\)上确定一点\(G\),使\(A\),\(E\),\(G\),\(F\)四点共面,并求此时\(C\)\({\,\!}_{1}\)\(G\)的长.