3.若$\overrightarrow{AB}$是非零向量,则下列等式正确的是( )
| A. | |$\overrightarrow{AB}$|=|$\overrightarrow{BA}$| | B. | |$\overrightarrow{AB}$|+|$\overrightarrow{BA}$|=0 | C. | $\overrightarrow{AB}$+$\overrightarrow{BA}$=0 | D. | $\overrightarrow{AB}$=$\overrightarrow{BA}$ |
2.四边形ABCD中,∠A=∠B=∠C=90°,下列条件能使这个四边形是正方形的是( )
| A. | ∠D=90° | B. | AB=CD | C. | BC=CD | D. | AC=BD |
20.
如图,已知在△ABC中,点D、E、F分别是AB、AC、BC的中点.下列结论不正确的是( )
| A. | $\overrightarrow{DE}$∥$\overrightarrow{BC}$ | B. | $\overrightarrow{AD}-\overrightarrow{AE}=\overrightarrow{DE}$ | C. | $\overrightarrow{DB}$=$-\overrightarrow{FE}$ | D. | $\overrightarrow{DB}+\overrightarrow{DE}+\overrightarrow{FE}=\overrightarrow{DE}$ |
18.
已知:如图,四边形ABCD是平行四边形,延长BA到点E,使AE=AB,联结ED、EC、AC.添加一个条件,能使四边形ACDE成为矩形的是( )
| A. | AC=CD | B. | AB=AD | C. | AD=AE | D. | BC=CE. |
17.
如图,已知?ABCD,在分别以四个顶点为起点和终点的向量中,向量$\overrightarrow{BD}$=( )
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| A. | $\overrightarrow{BA}$+$\overrightarrow{BC}$ | B. | $\overrightarrow{AB}$+$\overrightarrow{CB}$ | C. | $\overrightarrow{AB}$+$\overrightarrow{BC}$ | D. | $\overrightarrow{AB}$-$\overrightarrow{BC}$ |