13. 已知非零向量AB→\overrightarrow{AB}AB与AC→\overrightarrow{AC}AC满足(AB→∣AB→∣+AC→∣AC→∣)⋅BC→=0(\frac{\overrightarrow{AB}}{\vert {\overrightarrow{AB}}\vert }+\frac{\overrightarrow{AC}}{\vert {\overrightarrow{AC}}\vert })\centerdot \overrightarrow{BC}=0(∣AB∣AB+∣AC∣AC)⋅BC=0且AB→∣AB→∣⋅AC→∣AC→∣=12\frac{\overrightarrow{AB}}{\vert \overrightarrow{AB}\vert }\centerdot \frac{\overrightarrow{AC}}{\vert \overrightarrow{AC}\vert }=\frac{1}{2}∣AB∣AB⋅∣AC∣AC=21．则ΔABC\Delta ABCΔABC为((( )))