Question

Find the rectangular equation for the surface by eliminating the parameters from the vector-valued function. Identify the surface and sketch its graph. $$ \mathbf{r}(u, v)=2 \cos u \mathbf{i}+v \mathbf{j}+2 \sin u \mathbf{k} $$

Short answer

The rectangular equation for the given vector-valued function is \(x^2 + z^2 = 4\), y = v, and it represents a cylinder with radius 2 extending indefinitely along the y-axis.

Step-by-step solution

Express the vector equation in terms of x, y, and z

The vector function can be written as \(\mathbf{r}(u, v) = x \mathbf{i} + y \mathbf{j} + z \mathbf{k}\). This equates to the given vector \(\mathbf{r}(u, v) = 2 \cos u \mathbf{i} + v \mathbf{j} + 2 \sin u \mathbf{k}\). Thus, we have the system of equations x=2\cos u, y=v, z=2\sin u.

Eliminate the parameters

In order to remove the parameters, combine the expressions for x and z to get rid of u. We square both x and z terms and then add them together. We get \(x^2 + z^2 = 4 \cos^2 u + 4 \sin^2 u\). Using the Pythagorean identity \( \cos^2 u + \sin^2 u = 1\), we simplify the equation to \(x^2 + z^2 = 4\).

Form a Rectangular Equation

Now we assemble terms from step 1 and 2 into a complete rectangular equation: \(x^2 + z^2 = 4\), y = v. This is the rectangular form of the given vector equation.

Identify the surface

The equation \(x^2 + z^2 = 4\), y = v represents a cylinder. The reason it's a cylinder is due to the y variable being a single term (y = v). In the xz plane, the equation represents a circle with radius 2, and because there is no restriction on y, the cylinder extends infinitely in the y-axis, carrying this circular cross section along with it

Key concepts

Rectangular Equation

A rectangular equation is one in which relationships between the different variables are expressed in Cartesian coordinates, that is, as functions of the variables x, y, and z without any parameters. This is particularly handy in visualizing geometrical shapes in a three-dimensional coordinate system. When dealing with surfaces or curves, sometimes we are given vector-valued functions that include parameters. The example from the exercise demonstrates such a case, where we start with the parameters u and v.

During the transition from a vector-valued function to a rectangular equation, you're essentially translating the language of vectors into something you can plot on your usual Cartesian graph. It's like converting recipe instructions into a simple ingredient list. By finding the rectangular equation for the given vector-valued function, a student can identify and sketch the surface easily on a graph without having to think in terms of parameters.

Eliminating Parameters

Eliminating parameters is a technique used to transition from a parameterized equation to a more familiar rectangular form. In the exercise, the parameter u appears in both x and z components, linking these two in a specific way. To eliminate it, you look for a relationship between x and z that is independent of u, just like finding a common thread in a story that ties separate events together.

By squaring and adding the quantities related to x and z, students made use of the Pythagorean trigonometric identity, which is an invaluable trick in their playbook for dealing with trigonometric functions of a common parameter. This approach is especially effective when dealing with sinusoidal functions as in our example, leading to a simplified equation that no longer has the parameter u.

Cylindrical Surface

A cylindrical surface in mathematics can be thought of as the three-dimensional version of a circle. Just like how a straw's shape remains consistent no matter how tall it is, a cylindrical surface maintains a consistent cross-section along the length of one of its axes—in this case, the y-axis. In the rectangular equation \(x^2 + z^2 = 4\), y = v, it's clear that as y takes any value (the role of v), the circle defined by \(x^2 + z^2 = 4\) remains unchanged.

Such surfaces are understood as cylinders because if you slice them with a plane parallel to the xz-plane, you find a circle every time. This consistency is a hallmark of cylindrical surfaces and is why they appear in so many applications, from simple pipes to complex architectural designs. The equation gives us a way to represent all points on this three-dimensional object using the familiar context of a two-dimensional circle.

Three-Dimensional Coordinate System

Working within a three-dimensional coordinate system expands our playground for exploring geometric shapes from the flatland of the xy-plane to the vastness of xyz-space. In this environment, we can describe the location of any point through three coordinates: x, y, and z, which correspond to the dimensions of width, depth, and height. Students benefit from thinking of this like navigating a drone that can move north, east, or vertically at any moment.

Understanding how to visualize and manipulate shapes within this system is a fundamental skill in mathematics, physics, engineering, and many other disciplines. In our exercise example, the three-dimensional coordinate system allows us to express the invariant circular cross-section of the cylindrical surface in terms of x and z, while allowing the y-axis to run unrestricted, thus creating the visualization of the cylinder extending infinitely along the y-axis.