Given ABC is a triangle with BC = a, CA = b and AB = c.

Firstly, we need to prove.

From the triangle law of vector addition, we have

But, we know

Let, and

By taking cross product with, we get

We know the cross product of two vectors and forming an angle θ is

where is a unit vector perpendicular to and

Here, all the vectors are coplanar. So, the unit vector perpendicular to and is same as that of and.

Consider equation (I) again.

We have

By taking cross product with, we get

From (II) and (III), we get

Thus, and in ΔABC.