Ta có:
$MA^{2}$ + $MB^{2}$ + $MC^{2}$
= $\underset{$MA^{2}$}{\rightarrow}$ +$\underset{$MB^{2}$}{\rightarrow}$ +$\underset{$MC^{2}$}{\rightarrow}$
=$\underset{$MG^{2}$}{\rightarrow}$ + 2$\underset{MG}{\rightarrow}$.$\underset{GA}{\rightarrow}$
= 3 $MG^{2}$ + ($GA^{2}$ +$GB^{2}$ + $GC^{2}$)