Graph of the Function with Two Variables
Suppose we are given equation with two variables `f(x,y)=0`. If we draw all its solutions on coordinate plane, then we obtain some set of points on plane. This set is called graph of the equation `f(x,y)=0`.
For example, graph of the equation `y-x^2=0` is a parabola `y=x^2`; graph of the equation `y-x=0` is line `y=x` (bisector of First and Third Quadrants). Graph of the function `sqrt(x-1)+sqrt(y-2)=0` is one point (1;2), because coordinates of only this point satisfy equations.