\( \Rightarrow {\mkern 1mu} |\mathop {AB}\limits^ \to {\mkern 1mu} + {\mkern 1mu} \mathop {BC}\limits^ \to {\mkern 1mu} {\mkern 1mu} + {\mkern 1mu} {\mkern 1mu} \mathop {AC}\limits^ \to |{\mkern 1mu} {\mkern 1mu} = |2\mathop {AC}\limits^ \to {\mkern 1mu} |{\mkern 1mu} {\mkern 1mu} = {\mkern 1mu} {\mkern 1mu} 2{\mkern 1mu} {\mkern 1mu} |\mathop {AC}\limits^ \to {\mkern 1mu} |{\mkern 1mu} = {\mkern 1mu} {\mkern 1mu} 2a{\mkern 1mu} \)
\(\therefore {\mkern 1mu} {\mkern 1mu} n{\mkern 1mu} {\mkern 1mu} = {\mkern 1mu} {\mkern 1mu} 2\)