Let us given that,
If G is the centroid of a triangle ABC and let O be the position vector.
Then, let
OA=→a
OB=→b
OC=→c
OG=→g
We know that, using centroid formula
−−→OG=−−→OA−−−→+OB−−−→+OC3
→g=→a+→b+→c3
3→g=→a+→b+→c
→a+→b+→c=3→g......(1)
Now,
According to given question,
−−→GA+−−→GB+−−→GC
=(−−→OA−−−→OG)+(−−→OB−−−→OG)+(−−→OC−−−→OG)
=−−→OA+−−→OB+−−→OC−3−−→OG
→a+→b+→c−3→g by equation (1)
=3→g−3→g=0
Hence, it is complete solution.