A. \(\frac{{ - {a^2}}}{2}\)
B. \(\frac{{{c^2} + {b^2}}}{2}\)
C. \(\frac{{{c^2} + {b^2} + {a^2}}}{3}\)
D. \(-a^2\)
D
Ta có:
\(\overrightarrow {AM} .\overrightarrow {BC} = \frac{1}{2}(\overrightarrow {AB} + \overrightarrow {AC} ).\overrightarrow {BC} = \frac{1}{2}(\overrightarrow {AB} .\overrightarrow {BC} + \overrightarrow {AC} .\overrightarrow {BC} )\)
\( = \frac{1}{2}{\rm{[}}c.a.cos({180^0} - B) + b.a.cos({180^0} - C){\rm{]}}\)
\(= - (c.a.cosB + b.a.cosC)\)
\( = - (c.a.\frac{{{a^2} + {c^2} - {b^2}}}{{2ac}} + ab.\frac{{{a^2} + {b^2} - {c^2}}}{{2ab}})\)
\( = - \frac{1}{2}.2{a^2} = - {a^2}\)
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