0Vertical line test says that any line that is parallel to z-axis any line of form x=x0 and y=y0
intersect the graph z=f(x,y) of function at most one point
.because there is a point of intersection if and only if (x0, y0) is in the domain of the function
Otherwise, there is no existence of a point of intersection
As we Know from the definition of the graph of a function of two variables to each point (x,y) in the domain there
corresponds a point (x,y,z) on the surface and conversely to each position on the surface there corresponds a spot in the domain
hence a vertical line can intersect the graph at most once.
