Proof for parallelogram law of vector addition The Statement of Parallelogram law of vector addition is,

If two vectors are considered to be the adjacent sides of a parallelogram, then the resultant of two vectors is given by the vector that is a diagonal passing through the point of contact of two vectors.

But how do we justify that the resultant is along the diagonal? Is it based on experimental evidence, or is it something that can be proved? 
I know, the question might sound pretty obvious, but I'm new to this stuff. :)

If you hold a mirror to the two vectors, you get a mirror image of both. Turn the mirror image upside down, and the resultant vector of the upside down mirror image is the same magnitude as the original.  Combine the upside-down mirror image with the original and you have a parallelogram.  This is a proof of magnitude, only.
