5. 在正方体中ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1,已知OOO为A1C1A_{1}C_{1}A1C1中点,如图所示.(1)求证:B1C//B_{1}C//B1C//平面ODC1ODC_{1}ODC1;(2)求异面直线B1CB_{1}CB1C与ODODOD夹角大小.