Em giai the nay !!!\equiv
Goi trong tam cua tu dien la G
Suy ra →GA+→GB+→GC+→GD=→0
Dat GA=GB=GC=GD=x (voi x tinh dk !!!!)
Ta co →GAx+→GBx+→GCx+→GDx=→0
Va |→GAx|=|→GBx|=|→GCx|=|→GDx|=1
Do do'
PA+PB+PC+PD≥→PA.→GAx+→PB.→GBx+→PC.→GCx+→PD.→GDx=→PG(→GAx+→GBx+→GCx+→GDx)+GA+GB+GC+GD=4GA
Vay S dat min bang 4x khi P≡G