\(\dfrac{1}{2}\left| {\left[ {\overrightarrow {AB} ,\overrightarrow {AC} } \right]} \right|\)

\(\dfrac{1}{2}\left| {\left[ {\overrightarrow {AB} ,\overrightarrow {CA} } \right]} \right|\)        

\(\dfrac{1}{2}\left| {\left[ {\overrightarrow {BC} ,\overrightarrow {BA} } \right]} \right|\)

\(\dfrac{1}{2}\left| {\left[ {\overrightarrow {AB} ,\overrightarrow {AB} } \right]} \right|\)