The graph of is:
- defines the graph as that of a real-valued, 3-dimensional function
- defines the domain of the function
Level Curves & Contours
A curve on the surface for when stays constant is called a contour. If we take the example of a hemisphere, a contour would be one of the ‘rings’ on the hemisphere.
If that contour is drawn on the plane (i.e. ), it is known as a level curve. Again, with the hemisphere example, the level curve would looking at the ring from the top, downwards.
The key steps in drawing a graph of a function of two variables are:
- Draw some level curves.
- Draw the x-z and y-z cross sections. (known as traces)
- Draw the x, y, z axes.
- Draw the contours for the level curves you found earlier.
- Add the cross sections you drew earlier.
- Label any x, y, z axis intercepts and key points.
Examples
1: Hemisphere
Let . Sketch the graph of
- First, we need to find the maximal domain and range of . since square roots cannot exist on negative numbers. Thus the domain
- Now to find the range. We know that , since Since (squares cannot be negative), Which means the range is
- Drawing level curves: Let (Equation of a circle, ) The radius of the circle is bound between 0 and 1, since the range is . Hence, drawing the circles for different values of c: Sketching 3D Graphs E1a.excalidraw
- Now, to sketch the cross sections. At the plane, y = 0 and similarly at the plane, x = 0. at the plane is the equation of a semicircle. Doing the same process with the plane, is another semicircle. 1000
- Finally, we can combine these cross sections to create contours, and sketch the final graph: Sketching 3D Graphs 1c .excalidraw