véc tơ

H

hoangtrongminhduc

gọi G là trọng tâm ABC
ta có $\vec{AG}+\vec{BG}+\vec{CG}=\vec{0} \\ <=>\vec{AA'}+\vec{A'G}+\vec{BB'}+\vec{B'G}+\vec{CC'}+\vec{C'G}=0 \\ <=> (\vec{AA'}+\vec{BB'}+\vec{CC'})+\vec{A'G}+\vec{B'G}+\vec{C'G}=\vec{0} \\ <=> 2(\vec{AB}+\vec{Bc}+\vec{CA})+\vec{A'G}+\vec{B'G}+\vec{C'G}=\vec{0}<=>\vec{A'G}+\vec{B'G}+\vec{C'G}=\vec{0} dpcm$
 
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