# Section 1: Functions of Several Variables

### Instructions

• First, you should watch the concepts videos below explaining the topics in the section.
• Second, you should attempt to solve the exercises and then watch the videos explaining the exercises.
• Last, you should attempt to answer the self-assessment questions to determine how well you learned the material.
• When you have finished the material below, you can start on Section 2 or return to the main several variable calculus page.

### Concepts

• The definition of a function of two variables
• The graph of a function of two variables with domain $D$ and range $R$
• The level curves of a function of two variables

If you would like to see more videos on this topic, click the following link and see the related videos. Note the related videos at the link are not required viewing.

If you would like to see more videos on this topic, click the following link and see the related videos. Note the related videos at the link are not required viewing.

### Exercises

Directions: You should attempt to solve the problems first and then watch the video to see the solution.
1. Consider $f\left(x,y\right)=\mathrm{ln}\left(y-4x\right)$.
1. Evaluate $f\left(-1,3\right).$
2. Find the domain of $f\left(x,y\right)$ and sketch the domain of $f\left(x,y\right)$ in the $xy$-plane. ​

1. $$f(-1,3)=\ln (7)$$
2. Domain: $$\{ (x,y) \mid y-4x>0\}$$. See the video for a sketch.

If you would like to see more videos on this topic, click the following link and see the related videos. Note the related videos at the link are not required viewing.

2. Find the domain of $f\left(x,y\right)=\frac{\sqrt{9-{x}^{2}-{y}^{2}}}{x+y}$ and sketch the domain of $f\left(x,y\right)$ in the $xy$-plane.

Domain: $$\left\{ (x,y) \mid 9-x^2-y^2\geq 0 \; \text{ and } \; x+y\neq 0\right\}$$. See the video for a sketch.

If you would like to see more videos on this topic, click the following link and see the related videos. Note the related videos at the link are not required viewing.

3. Sketch the level curves for $f\left(x,y\right)=1-5x+y$ for $k=1,0,-1.$

See the video for the sketch.

If you would like to see more videos on this topic, click the following link and see the related videos. Note the related videos at the link are not required viewing.

4. Sketch the level curves for $f\left(x,y\right)=\sqrt{4-{x}^{2}-{y}^{2}}$ for $k=0,1,2.$

See the video for the sketch.

If you would like to see more videos on this topic, click the following link and see the related videos. Note the related videos at the link are not required viewing.

5. Sketch the level curve for $f\left(x,y\right)=\sqrt{{y}^{2}-{x}^{2}}$ for $k=4.$

See the video for the sketch.

If you would like to see more videos on this topic, click the following link and see the related videos. Note the related videos at the link are not required viewing.

### Self-Assessment Questions

Directions: The following questions are an assessment of your understanding of the material above. If you are not sure of the answers, you may need to rewatch the videos.
1. What are the necessary steps when finding the domain of a surface $z=f\left(x,y\right)$?
2. When finding the domain of a surface $z=f\left(x,y\right)$, how do we determine the set of all points $\left(x,y\right)$ where $f\left(x,y\right)$ is not defined?
3.  How do the level curves of a surface $z=f\left(x,y\right)$ help us visualize, and hence piece together,  the graph of a surface?
4. What is the relation between a level curve and a horizontal trace?