posted by Anonymous .
Triangle ABC is an equalateral triangle, with O it's centroid.
a) Show that vector OA + vector OB + vector OC = vector 0
Add components in direction of OA
|OA| - |OB| sin 30 - |OC| sin 30
but sin 30=1/2
Add components perpendicular to OA
|OC|cos 30 - |OB| cos 30 = 0