\(P=\overrightarrow{AB}\cdot\overrightarrow{AC}+\overrightarrow{AB}\cdot\overrightarrow{BC}\)
\(=\overrightarrow{AB}\cdot\overrightarrow{AC}-\overrightarrow{BA}\cdot\overrightarrow{BC}\)
\(=AB\cdot AC\cdot cos\left(\overrightarrow{AB},\overrightarrow{AC}\right)-BA\cdot BC\cdot cos\left(\overrightarrow{BA},\overrightarrow{BC}\right)\)
\(=AB\cdot AC\cdot cos60-BA\cdot BC\cdot cos60\)
\(=a\cdot a\cdot\dfrac{1}{2}-a\cdot a\cdot\dfrac{1}{2}\)
=0
=>Chọn A
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