a, G là trọng tâm=>HA+HB+HC=3HG

=>HA-3HG+HB+HC=0

=>HA-6HB+HB+HC=0 (vì HG=2HB)

b, AG+AH=a+b

=>AB+BG+AB+BH=a+b

=>2AB=a+b

AG+AH=a+b

=>AC+CG+AC+CH=a+b

=>2AC+CG+CH=a+b

=>2AC+(GA+GB)+(CA+AH)a+b (do GA+GB+GC=0)

=>AC-a+GB=a+b

=>AC=2a-GB

=>AC=2a-1/2GH

=>AC=2a-1/2(GA+AH)

=>AC=2a+1/2a-1/2b