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#30b796d2-e964-4244-b86d-975164aa1685基础填空题空间向量求角与距离立体几何

4. 在平行六面体ABCDA1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}中,点PP是线段BDBD上的一点,且PD=3PBPD=3PB,设A1A=a\overrightarrow{{A_1}A}=\overrightarrow{a}A1B1=b\overrightarrow{{A_1}{B_1}}=\overrightarrow{b}A1D1=c\overrightarrow{{A_1}{D_1}}=\overrightarrow{c},则PC1=(\overrightarrow{P{C_1}}=(  ))