Cho tứ diện ABCD. Chứng minh rằng:
a) \(\overrightarrow {AB} .\overrightarrow {CD} = \overrightarrow {AC} .\overrightarrow {CD} + \overrightarrow {BC} .\overrightarrow {DC} \);
b) \(\overrightarrow {AB} .\overrightarrow {CD} + \overrightarrow {AC} .\overrightarrow {DB} + \overrightarrow {AD} .\overrightarrow {BC} = 0\).
a) Ta có: \(\overrightarrow {AC} .\overrightarrow {CD} + \overrightarrow {BC} .\overrightarrow {DC} = \overrightarrow {AC} .\overrightarrow {CD} - \overrightarrow {BC} .\overrightarrow {CD} = \overrightarrow {CD} \left( {\overrightarrow {AC} + \overrightarrow {CB} } \right) = \overrightarrow {CD} .\overrightarrow {AB} \) (đpcm)
b) \(\overrightarrow {AB} .\overrightarrow {CD} + \overrightarrow {AC} .\overrightarrow {DB} + \overrightarrow {AD} .\overrightarrow {BC} = \overrightarrow {AB} .\overrightarrow {CD} + \left( {\overrightarrow {AB} + \overrightarrow {BC} } \right).\overrightarrow {DB} + \left( {\overrightarrow {AB} + \overrightarrow {BD} } \right).\overrightarrow {BC} \)
\( = \overrightarrow {AB} .\overrightarrow {CD} + \overrightarrow {AB} .\overrightarrow {DB} + \overrightarrow {BC} .\overrightarrow {DB} + \overrightarrow {AB} .\overrightarrow {BC} + \overrightarrow {BD} .\overrightarrow {BC} \)
\( = \overrightarrow {AB} .\left( {\overrightarrow {CD} + \overrightarrow {DB} + \overrightarrow {BC} } \right) + \left( {\overrightarrow {BC} .\overrightarrow {DB} + \overrightarrow {BD} .\overrightarrow {BC} } \right) = \overrightarrow {AB} .\left( {\overrightarrow {CB} + \overrightarrow {BC} } \right) + \overrightarrow {BC} \left( {\overrightarrow {DB} + \overrightarrow {BD} } \right) = 0\)