Chứng minh rằng:AB+BE+CF=AE+BF+CD

CHỗ CD phải là CB em nhé $\begin{array}{l} \overrightarrow {AB} + \overrightarrow {BE} + \overrightarrow {CF} = \overrightarrow {AE} + \overrightarrow {BF} + \overrightarrow {CB} \\ \Leftrightarrow \overrightarrow {AB} - \overrightarrow {AE} + \overrightarrow {BE} - \overrightarrow {BF} + \overrightarrow {CF} - \overrightarrow {CB} = \overrightarrow 0 \\ \Leftrightarrow \overrightarrow {EB} + \overrightarrow {FE} + \overrightarrow {BF} = \overrightarrow 0 \\ \Leftrightarrow \overrightarrow {EB} + \overrightarrow {BF} + \overrightarrow {FE} = \overrightarrow 0 \\ \Leftrightarrow \overrightarrow {EF} + \overrightarrow {FE} = \overrightarrow 0 \\ \Leftrightarrow \overrightarrow 0 = \overrightarrow 0 \left( {ld} \right) \end{array}$