Writing Equations in Three Dimensions

Learning Objectives

  • Write the distance formula in three dimensions.
  • Write the equations for simple planes and spheres.

Writing Equations in [latex]\mathbb{R}^3[/latex]

Now that we can represent points in space and find the distance between them, we can learn how to write equations of geometric objects such as lines, planes, and curved surfaces in [latex]\mathbb{R}^3[/latex]. First, we start with a simple equation. Compare the graphs of the equation [latex]x=0[/latex] in [latex]\mathbb{R}[/latex], [latex]\mathbb{R}^2[/latex], and [latex]\mathbb{R}^3[/latex] (Figure 1). From these graphs, we can see the same equation can describe a point, a line, or a plane.

This figure has three images. The first is a horizontal axis with a point drawn at 0. The second is the two dimensional Cartesian coordinate plane. The third is the 3-dimensional coordinate system. It is inside of a box and has a grid drawn at the y z-plane.

Figure 1. (a) In [latex]ℝ[/latex], the equation [latex]x=0[/latex] describes a single point. (b) In [latex]ℝ^2[/latex], the equation [latex]x=0[/latex] describes a line, the [latex]y[/latex]-axis. (c) In [latex]ℝ^3[/latex], the equation [latex]x=0[/latex] describes a plane, the [latex]yz[/latex]-plane.

In space, the equation [latex]x=0[/latex] describes all points [latex](0, y, z)[/latex]. This equation defines the [latex]yz[/latex]-plane. Similarly, the [latex]xy[/latex]-plane contains all points of the form [latex](x, y, 0)[/latex]. The equation [latex]z=0[/latex] defines the [latex]xy[/latex]-plane and the equation [latex]y=0[/latex] describes the [latex]xz[/latex]-plane (Figure 2).

This figure has two images. The first is the 3-dimensional coordinate system. It is inside of a box and has a grid drawn at the x y-plane. The second is the 3-dimensional coordinate system. It is inside of a box and has a grid drawn at the x z-plane.

Figure 2 (a) In space, the equation [latex]z=0[/latex] describes the [latex]xy[/latex]-plane. (b) All points in the [latex]xz[/latex]-plane satisfy the equation [latex]y=0[/latex].

Understanding the equations of the coordinate planes allows us to write an equation for any plane that is parallel to one of the coordinate planes. When a plane is parallel to the [latex]xy[/latex]-plane, for example, the [latex]z[/latex]-coordinate of each point in the plane has the same constant value. Only the [latex]x[/latex]– and [latex]y[/latex]-coordinates of points in that plane vary from point to point.

 

rule: equations of planes parallel to coordinate planes


  1. The plane in space that is parallel to the [latex]xy[/latex]-plane and contains point [latex](a, b, c)[/latex] can be represented by the equation [latex]z=c[/latex].
  2. The plane in space that is parallel to the [latex]xz[/latex]-plane and contains point [latex](a, b, c)[/latex] can be represented by the equation [latex]y=b[/latex].
  3. The plane in space that is parallel to the [latex]yz[/latex]-plane and contains point [latex](a, b, c)[/latex] can be represented by the equation [latex]x=a[/latex].

Example: writing equations of planes parallel to coordinate planes

  1. Write an equation of the plane passing through point [latex](3, 11, 7)[/latex] that is parallel to the [latex]yz[/latex]-plane.
  2. Find an equation of the plane passing through points [latex](6, -2, 9)[/latex], [latex](0, -2, 4)[/latex], and [latex](1, -2, -3)[/latex].

try it

Write an equation of the plane passing through point [latex](1, -6, -4)[/latex] that is parallel to the [latex]xy[/latex]-plane.

Try It

As we have seen, in [latex]\mathbb{R}^2[/latex] the equation [latex]x=5[/latex] describes the vertical line passing through point [latex](5, 0)[/latex]. This line is parallel to the [latex]y[/latex]-axis. In a natural extension, the equation [latex]x=5[/latex] in [latex]\mathbb{R}^3[/latex] describes the plane passing through point [latex](5, 0, 0)[/latex], which is parallel to the [latex]yz[/latex]-plane. Another natural extension of a familiar equation is found in the equation of a sphere.

definition


sphere is the set of all points in space equidistant from a fixed point, the center of the sphere (Figure 3), just as the set of all points in a plane that are equidistant from the center represents a circle. In a sphere, as in a circle, the distance from the center to a point on the sphere is called the radius.

This image is a sphere. It has center at (a, b, c) and has a radius represented with a broken line from the center point (a, b, c) to the edge of the sphere at (x, y, z). The radius is labeled “r.”

Figure 3. Each point [latex](x,y,z)[/latex] on the surface of a sphere is [latex]r[/latex] units away from the center [latex](a,b,c)[/latex].

The equation of a circle is derived using the distance formula in two dimensions. In the same way, the equation of a sphere is based on the three-dimensional formula for distance.

rule: equation of a sphere


The sphere with center [latex](a, b, c)[/latex] and radius [latex]r[/latex] can be represented by the equation

[latex](x - a)^2 +(y - b)^2+ (z - c)^2 =r^2[/latex]
.

This equation is known as the standard equation of a sphere.

Example: finding the equation of a sphere

Find the standard equation of the sphere with center [latex](10, 7, 4)[/latex] and point [latex](-1, 3, -2)[/latex] as shown in Figure 4.

This figure is a sphere centered on the point (10, 7, 4) of a 3-dimensional coordinate system. It has radius equal to the square root of 173 and passes through the point (-1, 3, -2).

Figure 4. The sphere centered at [latex](10,7,4)[/latex] containing point [latex](−1,3,−2)[/latex].

try it

Find the standard equation of the sphere with center [latex](-2, 4, 5)[/latex] containing point [latex](4, 4, -1)[/latex].

Example: finding the equation of a sphere

Let [latex]P=(-5, 2, 3)[/latex] and [latex]Q=(3, 4, -1)[/latex], and suppose line segment [latex]PQ[/latex] forms the diameter of a sphere (Figure 5). Find the equation of the sphere.

This figure is the 3-dimensional coordinate system. There are two points labeled. The first point is P = (-5, 2, 3). The second point is Q = (3, 4, -1). There is a line segment drawn between the two points.

Figure 5. Line segment [latex]PQ[/latex].

try it

Find the equation of the sphere with diameter [latex]PQ[/latex], where [latex]P=(2, -1, -3)[/latex] and [latex]Q=(-2, 5, -1)[/latex].

Watch the following video to see the worked solution to the above Try IT.

You can view the transcript for “CP 2.15” here (opens in new window)

Example: graphing other equations in three dimensions

Describe the set of points that satisfies [latex](x-4)(z-2)=0[/latex], and graph the set.

try it

Describe the set of points that satisfies [latex](y+2)(z-3)=0[/latex], and graph the set.

Example: graphing other equations in three dimensions

Describe the set of points in three-dimensional space that satisfies [latex](x+2)^{2}+(y-1)^{2}=4[/latex], and graph the set.

try it

Describe the set of points in three dimensional space that satisfies [latex]x^{2}+(z-2)^{2}=16[/latex], and graph the surface.

Watch the following video to see the worked solution to the above Try IT.

You can view the transcript for “CP 2.17” here (opens in new window)