ContourPlot[Exp[x y]/(y+3), {x, -1, 1}, {y, -1, 1}, Contours->9, ContourLabels->True]

ContourPlot[x^2+y^2, {x, -1, 1}, {y, -1, 1}, Contours->15, ContourLabels->True]

ContourPlot[x y, {x, -13, 13}, {y, -13, 13}, Contours->20, ContourLabels->True]

Plot3D[ x  y,  {x, -13, 13}, {y, -13, 13}]

ContourPlot3D[x^2+y^2-z^2==0,{x,-2,2},{y,-2,2},{z,-2,2},Contours->5]

ContourPlot3D[x^2+y^2==z,{x,-1,1},{y,-1,1},{z,-1,1}]

ContourPlot3D[x^3+y^2-z^2==0,{x,-2,2},{y,-2,2},{z,-2,2}]

ContourPlot[(x^4-y^2)/(x^4+y^2),{x,-4,4},{y,-4,4},ContourLabels->True]

In the graph above, can you determine the limit of f(x, y) as (x, y) -> (0,0) ?