math
posted by julie .
Prove that if a subset C of R*R is symmetric with respect to both the xaxis and yaxis, then it is symmetric with respect to the origin.

A reflection about both the x and y axes will move a point to the opposite side of the origin (180 degrees away), and the same distance away from it.
Hence there is symmetry of a set about the origin, if there is symmetry about the x and y axis.
This is probably no a "proof" in words that set theorists would prefer.