In fact, under the condition of OA+AB+AC=0, it is easy to judge that OBAC constitutes a parallelogram, because the vector addition law, OA+AB = OB; OA+AC=OC, so you can get OB=-AC=CA,? OC=-AB=BA，

Then there is the modulus OA=AB of the vector, and all four sides are equal. A diamond with a side length equal to a circle radius of 2.

When you ask the projection of CA in the direction of vector CB, the rhombic diagonals are perpendicular to each other, isn't it half of the long diagonal BC?

So L=√3