Ta có: \(\overrightarrow {AB} .\overrightarrow {AC} = \left| {\overrightarrow {AB} } \right|.\left| {\overrightarrow {AC} } \right|.\cos \left( {\overrightarrow {AB} ,\overrightarrow {AC} } \right)\)
Mà \(\left( {\overrightarrow {AB} ,\overrightarrow {AC} } \right) = \widehat {BAC}\)\( \Rightarrow \cos \left( {\overrightarrow {AB} ,\overrightarrow {AC} } \right) = \cos \widehat {BAC}\)
Lại có: \(\cos \widehat {BAC} = \frac{{{b^2} + {c^2} - {a^2}}}{{2bc}}\)(suy ra từ định lí cosin)
\(\begin{array}{l} \Rightarrow \overrightarrow {AB} .\overrightarrow {AC} = AB.AC.\frac{{{b^2} + {c^2} - {a^2}}}{{2bc}}\\ \Leftrightarrow \overrightarrow {AB} .\overrightarrow {AC} = c.b.\frac{{{b^2} + {c^2} - {a^2}}}{{2bc}}\\ \Leftrightarrow \overrightarrow {AB} .\overrightarrow {AC} = \frac{{{b^2} + {c^2} - {a^2}}}{2}\end{array}\)
