Ta có: tứ giác \(ABCD\) là hình bình hành nên: \(\left\{ \matrix{\overrightarrow {AB} = \overrightarrow {DC} \hfill \cr \overrightarrow {AD} = \overrightarrow {BC} \hfill \cr} \right.\)
\(\overrightarrow {AC} + \overrightarrow {BD} = \overrightarrow {AB} +\overrightarrow {AD}+ \overrightarrow {BC} + \overrightarrow {BA}\)\( = 2\overrightarrow {BC}\)
\(\overrightarrow {AC} + \overrightarrow {BC} = \overrightarrow {AB} + \overrightarrow {BC} + \overrightarrow {BC}\)\( = \overrightarrow {AB} + 2\overrightarrow {BC} \ne \overrightarrow {AB} \)
\(\overrightarrow {AC} - \overrightarrow {BD} = \overrightarrow {AB} + \overrightarrow {BC} - \overrightarrow {BC} - \overrightarrow {CD}\)\(= \overrightarrow {AB} + \overrightarrow {DC} = 2\overrightarrow {AB} \ne 2\overrightarrow {CD} \)
\(\overrightarrow {AC} - \overrightarrow {AD} = \overrightarrow {AB} + \overrightarrow {BC} - \overrightarrow {BC} \)\(= \overrightarrow {AB} \ne \overrightarrow {CD} \)
Vậy A đúng.
