AO*BC
=AO*(BO+OC)
=AO*BO+AO*OC
=OA*OB-OA*OC
=|OA|*|OB|*cos∠AOB-|OA|*|OC|*cos∠AOC
=|OA|*|OB|*(|OA|^2+|OB|^2-|AB|^2)/(2|OA|*|OB|)-|OA|*|OC|*(|OA|^2+|OC|^2-|AC|^2)/(2|OA|*|OC|)
=(|OA|^2+|OB|^2-|AB|^2)/2-(|OA|^2+|OC|^2-|AC|^2)/2
=(|AC|^2-|AB|^2)/2
=(16-4)/2
=6
其中|OA|=|OB|=|OC|,因为O是外接圆圆心
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