Three-Dimensional Coordinate Systems

```Jim Lambers
MAT 169
Fall Semester 2009-10
Lecture 17 Notes
These notes correspond to Section 10.1 in the text.
Three-Dimensional Coordinate Systems
Over the course of the next several lectures, we will learn how to work with locations and directions
in three-dimensional space, in order to easily describe objects such as lines, planes and curves. This
will set the stage for the study of functions of two variables, the graphs of which are surfaces in
space.
Points in Three-Dimensional Space
Previously, we have identiο¬ed a point in the π₯π¦-plane by an ordered pair that consists of two real
numbers, an π₯-coordinate and π¦-coordinate, which denote signed distances along the π₯-axis and
π¦-axis, respectively, from the origin, which is the point (0, 0). These axes, which are collectively
referred to as the coordinate axes, divided the plane into four quadrants.
We now generalize these concepts to three-dimensional space, or π₯π¦π§-space. In this space, a
point is represented by an ordered triple (π₯, π¦, π§) that consists of three numbers, an π₯-coordiante, a
π¦-coordinate, and a π§-coordinate. As in the two-dimensional π₯π¦-plane, these coordinates indicate
the signed distance along the coordinate axes, the π₯-axis, π¦-axis and π§-axis, respectively, from the
origin, denoted by π, which has coordinates (0, 0, 0). There is a one-to-one correspondence between
a point in π₯π¦π§-space and a triple in β3 , which is the set of all ordered triples of real numbers. This
correspondence is known as a three-dimensional rectangular coordinate system.
Example Figure 1 displays the point (2, 3, 1) in π₯π¦π§-space, denoted by the letter π , along with its
projections onto the coordinate planes (described below). The origin is denoted by the letter π. β‘
Planes in Three-Dimensional Space
Unlike two-dimensional space, which consists of a single plane, the π₯π¦-plane, three-dimensional
space contains inο¬nitely many planes, just as two-dimensional space consists of inο¬nitely many
lines. Three planes are of particular importance: the π₯π¦-plane, which contains the π₯- and π¦-axes;
the π¦π§-plane, which contains the π¦- and π§-axes; and the π₯π§-plane, which contains the π₯- and π§-axes.
Alternatively, the π₯π¦-plane can be described as the set of all points (π₯, π¦, π§) for which π§ = 0.
Similarly, the π¦π§-plane is the set of all points of the form (0, π¦, π§), while the π₯π§-plane is the set of
all points of the form (π₯, 0, π§).
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Figure 1: The point (2, 3, 1) in π₯π¦π§-space, denoted by the letter π . The origin is denoted by
the letter π. The projections of π onto the coordinate planes are indicated by the diamonds.
The dashed lines are line segments perpendicular to the coordinate planes that connect π to its
projections.
Just as the π₯-axis and π¦-axis divide the π₯π¦-plane into four quadrants, these three planes divide
π₯π¦π§-space into eight octants. Within each octant, all π₯-coordiantes have the same sign, as do all
π¦-coordinates, and all π§-coordinates. In particular, the ο¬rst octant is the octant in which all three
coordinates are positive.
Plotting Points in π₯π¦π§-space
Graphing in π₯π¦π§-space can be diο¬cult because, unlike graphing in the π₯π¦-plane, depth perception
is required. To simplify plotting of points, one can make use of projections onto the coordinate
planes. The projection of a point (π₯, π¦, π§) onto the π₯π¦-plane is obtained by connecting the point to
the π₯π¦-plane by a line segment that is perpendicular to the plane, and computing the intersection
of the line segment with the plane.
Later, we will learn more about how to compute projections of points onto planes, but in this
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relatively simple case, it follows from our working deο¬nition that the projection of the point (π₯, π¦, π§)
onto the π₯π¦-plane is the point (π₯, π¦, 0). Similarly, the projection of this point onto the π¦π§-plane is
the point (0, π¦, π§), and the projection of this point onto the π₯π§-plane is the point (π₯, 0, π§). Figure 1
illustrates these projections, and how they can be used to plot a point in π₯π¦π§-space. One can ο¬rst
plot the point’s projections, which is similar to the task of plotting points in the π₯π¦-plane, and then
use line segments originating from these projections and perpendicular to the coordinate planes to
“locate” the point in π₯π¦π§-space.
The Distance Formula
The distance between two points π1 = (π₯1 , π¦1 ) and π2 = (π₯2 , π¦2 ) in the π₯π¦-plane is given by the
distance formula,
√
π(π1 , π2 ) = (π₯2 − π₯1 )2 + (π¦2 − π¦1 )2 .
Similarly, the distance between two points π1 = (π₯1 , π¦1 , π§1 ) and π2 = (π₯2 , π¦2 , π§2 ) in π₯π¦π§-space is
given by the following generalization of the distance formula,
√
π(π1 , π2 ) = (π₯2 − π₯1 )2 + (π¦2 − π¦1 )2 + (π§2 − π§1 )2 .
This can be proved by repeated application of the Pythagorean Theorem.
Example The distance between π1 = (2, 3, 1) and π2 = (8, −5, 0) is
√
√
√
π(π1 , π2 ) = (8 − 2)2 + (−5 − 3)2 + (0 − 1)2 = 36 + 64 + 1 = 101 ≈ 10.05.
β‘
Equations of Surfaces
In two dimensions, the solution set of a single equation involving the coordinates π₯ and/or π¦ is a
curve. In three dimensions, the solution set of an equation involving π₯, π¦ and/or π§ is a surface.
Example The equation π§ = 3 describes a plane that is parallel to the π₯π¦-plane, and is 3 units
“above” it; that is, it lies 3 units along the positive π§-axis from the π₯π¦-plane. On the other hand,
the equation π₯ = π¦ describes a plane consisting of all points whose π₯- and π¦-coordinates are equal.
It is not parallel to any coordinate plane, but it contains the π§-axis, which consists of all points
whose π₯- and π¦-coordinates are both zero, and it intersects the π₯π¦-plane at the line π¦ = π₯. β‘
The equation of a sphere with center πΆ = (β, π, β) and radius π is
(π₯ − β)2 + (π¦ − π)2 + (π§ − β)2 = π2 .
The unit sphere has center π = (0, 0, 0) and radius 1:
π₯2 + π¦ 2 + π§ 2 = 1.
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We now illustrate how to work with equations of spheres.
Example The equation of a sphere with center πΆ = (−3, −1, 1) and radius π = 10 is
(π₯ − (−3))2 + (π¦ − (−1))2 + (π§ − 1)2 = 102 ,
or
(π₯ + 3)2 + (π¦ + 1)2 + (π§ − 1)2 = 100.
Expanding, we obtain
π₯2 + π¦ 2 + π§ 2 + 6π₯ + 2π¦ − 2π§ = 89,
which obscures the center and radius, but it is still possible to detect that the equation represents
a sphere, due to the fact that the π₯2 , π¦ 2 and π§ 2 terms have equal coeο¬cients. β‘
Example The equation
4π₯2 + 4π¦ 2 + 4π§ 2 − 8π₯ − 16π¦ − 16 = 0
describes a sphere, as can be seen by the equal coeο¬cients in front of the π₯2 , π¦ 2 and π§ 2 . To
determine the radius and center of the sphere, we complete the square in π₯ and π¦:
0 = 4π₯2 + 4π¦ 2 + 4π§ 2 − 8π₯ − 16π¦ − 16
= 4(π₯2 − 2π₯) + 4(π¦ 2 − 4π¦) + 4π§ 2 − 16
= 4(π₯2 − 2π₯ + 1 − 1) + 4(π¦ 2 − 4π¦ + 4 − 4) + 4π§ 2 − 16
= 4[(π₯ − 1)2 − 1] + 4[(π¦ − 2)2 − 4] + 4π§ 2 − 16
= 4(π₯ − 1)2 + 4(π¦ − 2)2 + 4π§ 2 − 36.
Rearranging, we obtain the standard form of the equation of the sphere:
(π₯ − 1)2 + (π¦ − 2)2 + π§ 2 = 9,
which reveals that the center is at the point πΆ = (1, 2, 0), and the radius is π = 3. β‘
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Summary
β The three-dimensional rectangular coordinate system is the one-to-one correspondence between each point π in three-dimensional space, or π₯π¦π§-space, and an ordered triple (π₯, π¦, π§)
in β3 . The numbers π₯, π¦ and π§ are the π₯-, π¦- and π§-coordinates of π . The origin π is the
point with coordinates (0, 0, 0).
β The coordinate planes are: the π₯π¦-plane, the set of all points whose π§-coordinate is zero; the
π¦π§-plane, the set of all points whose π₯-coordinate is zero; and the π₯π§-plane, the set of all
points whose π¦-coordinate is zero.
β The projection of a point π = (π₯, π¦, π§) onto the π₯π¦-plane is the point (π₯, π¦, 0). The projection
of π onto the π¦π§-plane is the point (0, π¦, π§). The projection of π onto the π₯π§-plane is the
point (π₯, 0, π§).
β The distance formula states that the distance between two points in π₯π¦π§-space is the square
root of the sum of the squares of the diο¬erences between corresponding coordinates. That
is, given π1 √
= (π₯1 , π¦1 , π§1 ) and π2 = (π₯2 , π¦2 , π§2 ), the distance between π1 and π2 is given by
π(π1 , π2 ) = (π₯2 − π₯1 )2 + (π¦2 − π¦1 )2 + (π§2 − π§1 )2 .
β The equation of a sphere with center πΆ = (π₯0 , π¦0 , π§0 ) and radius π is (π₯ − π₯0 )2 + (π¦ − π¦0 )2 +
(π§ − π§0 )2 = π2 .
β An equation in which π₯2 , π¦ 2 and π§ 2 have the same coeο¬cients describes a sphere; the center
and radius can be determined by completing the square in π₯, π¦ and π§.
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