Vectors in Space

# 12 Equations of Lines and Planes in Space

### Learning Objectives

- Write the vector, parametric, and symmetric of a line through a given point in a given direction, and a line through two given points.
- Find the distance from a point to a given line.
- Write the vector and scalar equations of a plane through a given point with a given normal.
- Find the distance from a point to a given plane.
- Find the angle between two planes.

By now, we are familiar with writing equations that describe a line in two dimensions. To write an equation for a line, we must know two points on the line, or we must know the direction of the line and at least one point through which the line passes. In two dimensions, we use the concept of slope to describe the orientation, or direction, of a line. In three dimensions, we describe the direction of a line using a vector parallel to the line. In this section, we examine how to use equations to describe lines and planes in space.

### Equations for a Line in Space

Let’s first explore what it means for two vectors to be parallel. Recall that parallel vectors must have the same or opposite directions. If two nonzero vectors, and are parallel, we claim there must be a scalar, such that If and have the same direction, simply choose If and have opposite directions, choose Note that the converse holds as well. If for some scalar then either and have the same direction or opposite directions so and are parallel. Therefore, two nonzero vectors and are parallel if and only if for some scalar By convention, the zero vector is considered to be parallel to all vectors.

As in two dimensions, we can describe a line in space using a point on the line and the direction of the line, or a parallel vector, which we call the direction vector ((Figure)). Let be a line in space passing through point Let be a vector parallel to Then, for any point on line we know that is parallel to Thus, as we just discussed, there is a scalar, such that which gives

Using vector operations, we can rewrite (Figure) as

Setting and we now have the vector equation of a line:

Equating components, (Figure) shows that the following equations are simultaneously true: and If we solve each of these equations for the component variables we get a set of equations in which each variable is defined in terms of the parameter *t* and that, together, describe the line. This set of three equations forms a set of parametric equations of a line:

If we solve each of the equations for assuming are nonzero, we get a different description of the same line:

Because each expression equals *t*, they all have the same value. We can set them equal to each other to create symmetric equations of a line:

We summarize the results in the following theorem.

A line parallel to vector and passing through point can be described by the following parametric equations:

If the constants are all nonzero, then can be described by the symmetric equation of the line:

The parametric equations of a line are not unique. Using a different parallel vector or a different point on the line leads to a different, equivalent representation. Each set of parametric equations leads to a related set of symmetric equations, so it follows that a symmetric equation of a line is not unique either.

Find parametric and symmetric equations of the line passing through points and

First, identify a vector parallel to the line:

Use either of the given points on the line to complete the parametric equations:

Solve each equation for to create the symmetric equation of the line:

Find parametric and symmetric equations of the line passing through points and

Possible set of parametric equations:

related set of symmetric equations:

Start by finding a vector parallel to the line.

Sometimes we don’t want the equation of a whole line, just a line segment. In this case, we limit the values of our parameter For example, let and be points on a line, and let and be the associated position vectors. In addition, let We want to find a vector equation for the line segment between and Using as our known point on the line, and as the direction vector equation, (Figure) gives

Using properties of vectors, then

Thus, the vector equation of the line passing through and is

Remember that we didn’t want the equation of the whole line, just the line segment between and Notice that when we have and when we have Therefore, the vector equation of the line segment between and is

Going back to (Figure), we can also find parametric equations for this line segment. We have

Then, the parametric equations are

Find parametric equations of the line segment between the points and

By (Figure), we have

Working with each component separately, we get

and

Therefore, the parametric equations for the line segment are

### Distance between a Point and a Line

We already know how to calculate the distance between two points in space. We now expand this definition to describe the distance between a point and a line in space. Several real-world contexts exist when it is important to be able to calculate these distances. When building a home, for example, builders must consider “setback” requirements, when structures or fixtures have to be a certain distance from the property line. Air travel offers another example. Airlines are concerned about the distances between populated areas and proposed flight paths.

Let be a line in the plane and let be any point not on the line. Then, we define distance from to as the length of line segment where is a point on such that is perpendicular to ((Figure)).

When we’re looking for the distance between a line and a point in space, (Figure) still applies. We still define the distance as the length of the perpendicular line segment connecting the point to the line. In space, however, there is no clear way to know which point on the line creates such a perpendicular line segment, so we select an arbitrary point on the line and use properties of vectors to calculate the distance. Therefore, let be an arbitrary point on line and let be a direction vector for ((Figure)).

By (Figure), vectors and form two sides of a parallelogram with area Using a formula from geometry, the area of this parallelogram can also be calculated as the product of its base and height:

We can use this formula to find a general formula for the distance between a line in space and any point not on the line.

Let be a line in space passing through point with direction vector If is any point not on then the distance from to is

Find the distance between t point and line

From the symmetric equations of the line, we know that vector is a direction vector for the line. Setting the symmetric equations of the line equal to zero, we see that point lies on the line. Then,

To calculate the distance, we need to find

Therefore, the distance between the point and the line is ((Figure))

Find the distance between point and the line with parametric equations

Find a vector with initial point and a terminal point on the line, and then find a direction vector for the line.

### Relationships between Lines

Given two lines in the two-dimensional plane, the lines are equal, they are parallel but not equal, or they intersect in a single point. In three dimensions, a fourth case is possible. If two lines in space are not parallel, but do not intersect, then the lines are said to be skew lines ((Figure)).

To classify lines as parallel but not equal, equal, intersecting, or skew, we need to know two things: whether the direction vectors are parallel and whether the lines share a point ((Figure)).

For each pair of lines, determine whether the lines are equal, parallel but not equal, skew, or intersecting.

- Line has direction vector line has direction vector Because the direction vectors are not parallel vectors, the lines are either intersecting or skew. To determine whether the lines intersect, we see if there is a point, that lies on both lines. To find this point, we use the parametric equations to create a system of equalities:

By the first equation, Substituting into the second equation yields

Substitution into the third equation, however, yields a contradiction:

There is no single point that satisfies the parametric equations for simultaneously. These lines do not intersect, so they are skew (see the following figure).

- Line
*L*has direction vector and passes through the origin, Line has a different direction vector, so these lines are not parallel or equal. Let represent the parameter for line and let represent the parameter for_{1}

Solve the system of equations to find and If we need to find the point of intersection, we can substitute these parameters into the original equations to get (see the following figure).

- Lines and have equivalent direction vectors: These two lines are parallel (see the following figure).

Describe the relationship between the lines with the following parametric equations:

These lines are skew because their direction vectors are not parallel and there is no point that lies on both lines.

Start by identifying direction vectors for each line. Is one a multiple of the other?

### Equations for a Plane

We know that a line is determined by two points. In other words, for any two distinct points, there is exactly one line that passes through those points, whether in two dimensions or three. Similarly, given any three points that do not all lie on the same line, there is a unique plane that passes through these points. Just as a line is determined by two points, a plane is determined by three.

This may be the simplest way to characterize a plane, but we can use other descriptions as well. For example, given two distinct, intersecting lines, there is exactly one plane containing both lines. A plane is also determined by a line and any point that does not lie on the line. These characterizations arise naturally from the idea that a plane is determined by three points. Perhaps the most surprising characterization of a plane is actually the most useful.

Imagine a pair of orthogonal vectors that share an initial point. Visualize grabbing one of the vectors and twisting it. As you twist, the other vector spins around and sweeps out a plane. Here, we describe that concept mathematically. Let be a vector and be a point. Then the set of all points such that is orthogonal to forms a plane ((Figure)). We say that is a normal vector, or perpendicular to the plane. Remember, the dot product of orthogonal vectors is zero. This fact generates the vector equation of a plane: Rewriting this equation provides additional ways to describe the plane:

Given a point and vector the set of all points satisfying the equation forms a plane. The equation

is known as the vector equation of a plane.

The scalar equation of a plane containing point with normal vector is

This equation can be expressed as where This form of the equation is sometimes called the general form of the equation of a plane.

As described earlier in this section, any three points that do not all lie on the same line determine a plane. Given three such points, we can find an equation for the plane containing these points.

Write an equation for the plane containing points and in both standard and general forms.

To write an equation for a plane, we must find a normal vector for the plane. We start by identifying two vectors in the plane:

The cross product is orthogonal to both and so it is normal to the plane that contains these two vectors:

Thus, and we can choose any of the three given points to write an equation of the plane:

The scalar equations of a plane vary depending on the normal vector and point chosen.

Find an equation of the plane that passes through point and contains the line given by

Symmetric equations describe the line that passes through point parallel to vector (see the following figure). Use this point and the given point, to identify a second vector parallel to the plane:

Use the cross product of these vectors to identify a normal vector for the plane:

The scalar equations for the plane are and

Find an equation of the plane containing the lines and

or

The cross product of the lines’ direction vectors gives a normal vector for the plane.

Now that we can write an equation for a plane, we can use the equation to find the distance between a point and the plane. It is defined as the shortest possible distance from to a point on the plane.

Just as we find the two-dimensional distance between a point and a line by calculating the length of a line segment perpendicular to the line, we find the three-dimensional distance between a point and a plane by calculating the length of a line segment perpendicular to the plane. Let bet the point in the plane such that is orthogonal to the plane, and let be an arbitrary point in the plane. Then the projection of vector onto the normal vector describes vector as shown in (Figure).

Suppose a plane with normal vector passes through point The distance from the plane to a point not in the plane is given by

Find the distance between point and the plane given by (see the following figure).

The coefficients of the plane’s equation provide a normal vector for the plane: To find vector we need a point in the plane. Any point will work, so set to see that point lies in the plane. Find the component form of the vector from

Apply the distance formula from (Figure):

Find the distance between point and the plane given by

Point lies on the plane.

### Parallel and Intersecting Planes

We have discussed the various possible relationships between two lines in two dimensions and three dimensions. When we describe the relationship between two planes in space, we have only two possibilities: the two distinct planes are parallel or they intersect. When two planes are parallel, their normal vectors are parallel. When two planes intersect, the intersection is a line ((Figure)).

We can use the equations of the two planes to find parametric equations for the line of intersection.

Find parametric and symmetric equations for the line formed by the intersection of the planes given by and (see the following figure).

Note that the two planes have nonparallel normals, so the planes intersect. Further, the origin satisfies each equation, so we know the line of intersection passes through the origin. Add the plane equations so we can eliminate the one of the variables, in this case,

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This gives us We substitute this value into the first equation to express in terms of

We now have the first two variables, and in terms of the third variable, Now we define in terms of To eliminate the need for fractions, we choose to define the parameter as Then, Substituting the parametric representation of back into the other two equations, we see that the parametric equations for the line of intersection are The symmetric equations for the line are

Find parametric equations for the line formed by the intersection of planes and

Add the two equations, then express in terms of Then, express in terms of

In addition to finding the equation of the line of intersection between two planes, we may need to find the angle formed by the intersection of two planes. For example, builders constructing a house need to know the angle where different sections of the roof meet to know whether the roof will look good and drain properly. We can use normal vectors to calculate the angle between the two planes. We can do this because the angle between the normal vectors is the same as the angle between the planes. (Figure) shows why this is true.

We can find the measure of the angle *θ* between two intersecting planes by first finding the cosine of the angle, using the following equation:

We can then use the angle to determine whether two planes are parallel or orthogonal or if they intersect at some other angle.

Determine whether each pair of planes is parallel, orthogonal, or neither. If the planes are intersecting, but not orthogonal, find the measure of the angle between them. Give the answer in radians and round to two decimal places.

- The normal vectors for these planes are and These two vectors are scalar multiples of each other. The normal vectors are parallel, so the planes are parallel.
- The normal vectors for these planes are and Taking the dot product of these vectors, we have

The normal vectors are orthogonal, so the corresponding planes are orthogonal as well. - The normal vectors for these planes are and

The angle between the two planes is rad, or approximately

Find the measure of the angle between planes and Give the answer in radians and round to two decimal places.

rad

Use the coefficients of the variables in each equation to find a normal vector for each plane.

When we find that two planes are parallel, we may need to find the distance between them. To find this distance, we simply select a point in one of the planes. The distance from this point to the other plane is the distance between the planes.

Previously, we introduced the formula for calculating this distance in (Figure):

where is a point on the plane, is a point not on the plane, and is the normal vector that passes through point Consider the distance from point to plane Let be any point in the plane. Substituting into the formula yields

We state this result formally in the following theorem.

Let be a point. The distance from to plane is given by

Find the distance between the two parallel planes given by and

Point lies in the first plane. The desired distance, then, is

Find the distance between parallel planes and

Set to find a point on the first plane.

Finding the distance from a point to a line or from a line to a plane seems like a pretty abstract procedure. But, if the lines represent pipes in a chemical plant or tubes in an oil refinery or roads at an intersection of highways, confirming that the distance between them meets specifications can be both important and awkward to measure. One way is to model the two pipes as lines, using the techniques in this chapter, and then calculate the distance between them. The calculation involves forming vectors along the directions of the lines and using both the cross product and the dot product.

The symmetric forms of two lines, and are

You are to develop a formula for the distance between these two lines, in terms of the values The distance between two lines is usually taken to mean the minimum distance, so this is the length of a line segment or the length of a vector that is perpendicular to both lines and intersects both lines.

- First, write down two vectors, and that lie along and respectively.
- Find the cross product of these two vectors and call it This vector is perpendicular to and hence is perpendicular to both lines.
- From vector form a unit vector in the same direction.
- Use symmetric equations to find a convenient vector that lies between any two points, one on each line. Again, this can be done directly from the symmetric equations.
- The dot product of two vectors is the magnitude of the projection of one vector onto the other—that is, where is the angle between the vectors. Using the dot product, find the projection of vector found in step onto unit vector found in step 3. This projection is perpendicular to both lines, and hence its length must be the perpendicular distance between them. Note that the value of may be negative, depending on your choice of vector or the order of the cross product, so use absolute value signs around the numerator.
- Check that your formula gives the correct distance of between the following two lines:

- Is your general expression valid when the lines are parallel? If not, why not? (
*Hint:*What do you know about the value of the cross product of two parallel vectors? Where would that result show up in your expression for - Demonstrate that your expression for the distance is zero when the lines intersect. Recall that two lines intersect if they are not parallel and they are in the same plane. Hence, consider the direction of and What is the result of their dot product?
- Consider the following application. Engineers at a refinery have determined they need to install support struts between many of the gas pipes to reduce damaging vibrations. To minimize cost, they plan to install these struts at the closest points between adjacent skewed pipes. Because they have detailed schematics of the structure, they are able to determine the correct lengths of the struts needed, and hence manufacture and distribute them to the installation crews without spending valuable time making measurements.

The rectangular frame structure has the dimensions (height, width, and depth). One sector has a pipe entering the lower corner of the standard frame unit and exiting at the diametrically opposed corner (the one farthest away at the top); call this A second pipe enters and exits at the two different opposite lower corners; call this ((Figure)).

Write down the vectors along the lines representing those pipes, find the cross product between them from which to create the unit vector define a vector that spans two points on each line, and finally determine the minimum distance between the lines. (Take the origin to be at the lower corner of the first pipe.) Similarly, you may also develop the symmetric equations for each line and substitute directly into your formula.

### Key Concepts

- In three dimensions, the direction of a line is described by a direction vector. The vector equation of a line with direction vector passing through point is where is the position vector of point This equation can be rewritten to form the parametric equations of the line: and The line can also be described with the symmetric equations
- Let be a line in space passing through point with direction vector If is any point not on then the distance from to is
- In three dimensions, two lines may be parallel but not equal, equal, intersecting, or skew.
- Given a point and vector the set of all points satisfying equation forms a plane. Equation is known as the
*vector equation of a plane.* - The scalar equation of a plane containing point with normal vector is This equation can be expressed as where This form of the equation is sometimes called the
*general form of the equation of a plane*. - Suppose a plane with normal vector
**n**passes through point The distance from the plane to point not in the plane is given by

- The normal vectors of parallel planes are parallel. When two planes intersect, they form a line.
- The measure of the angle between two intersecting planes can be found using the equation: where and are normal vectors to the planes.
- The distance from point to plane is given by

### Key Equations

**Vector Equation of a Line****Parametric Equations of a Line**

and**Symmetric Equations of a Line****Vector Equation of a Plane****Scalar Equation of a Plane****Distance between a Plane and a Point**

In the following exercises, points and are given. Let be the line passing through points and

- Find the vector equation of line
- Find parametric equations of line
- Find symmetric equations of line
- Find parametric equations of the line segment determined by and

a. b. c. d.

a. b. c. d.

For the following exercises, point and vector are given. Let be the line passing through point with direction

- Find parametric equations of line
- Find symmetric equations of line
- Find the intersection of the line with the
*xy*-plane.

a. b. c.

where and

a. b. c. The line does not intersect the *xy*-plane.

where and

For the following exercises, line is given.

- Find point that belongs to the line and direction vector of the line. Express in component form.
- Find the distance from the origin to line

a. b.

Find the distance between point and the line of symmetric equations

Find the distance between point and the line of parametric equations

For the following exercises, lines and are given.

- Verify whether lines and are parallel.
- If the lines and are parallel, then find the distance between them.

a. Parallel; b.

Show that the line passing through points and is perpendicular to the line with equation

Are the lines of equations and perpendicular to each other?

Find the point of intersection of the lines of equations and

Find the intersection point of the *x*-axis with the line of parametric equations

For the following exercises, lines and are given. Determine whether the lines are equal, parallel but not equal, skew, or intersecting.

and

The lines are skew.

and

and

The lines are equal.

and

Consider line of symmetric equations and point

- Find parametric equations for a line parallel to that passes through point
- Find symmetric equations of a line skew to and that passes through point
- Find symmetric equations of a line that intersects and passes through point

a. b. For instance, the line passing through with direction vector c. For instance, the line passing through and point that belongs to is a line that intersects;

Consider line of parametric equations

- Find parametric equations for a line parallel to that passes through the origin.
- Find parametric equations of a line skew to that passes through the origin.
- Find symmetric equations of a line that intersects and passes through the origin.

For the following exercises, point and vector are given.

- Find the scalar equation of the plane that passes through and has normal vector
- Find the general form of the equation of the plane that passes through and has normal vector

a. b.

a. b.

For the following exercises, the equation of a plane is given.

- Find normal vector to the plane. Express using standard unit vectors.
- Find the intersections of the plane with the axes of coordinates.
- Sketch the plane.

**[T]**

a. b. and

c.

a. b.

c.

Given point and vector find point on the *x*-axis such that and are orthogonal.

Show there is no plane perpendicular to that passes through points and

Find parametric equations of the line passing through point that is perpendicular to the plane of equation

Find symmetric equations of the line passing through point that is perpendicular to the plane of equation

Show that line is parallel to plane

Find the real number such that the line of parametric equations is parallel to the plane of equation

For the following exercises, points are given.

- Find the general equation of the plane passing through
- Write the vector equation of the plane at a., where is an arbitrary point of the plane.
- Find parametric equations of the line passing through the origin that is perpendicular to the plane passing through

and

a. b. c.

and

Consider the planes of equations and

- Show that the planes intersect.
- Find symmetric equations of the line passing through point that is parallel to the line of intersection of the planes.

a. Answers may vary; b.

Consider the planes of equations and

- Show that the planes intersect.
- Find parametric equations of the line passing through point that is parallel to the line of intersection of the planes.

Find the scalar equation of the plane that passes through point and is perpendicular to the line of intersection of planes and

Find the general equation of the plane that passes through the origin and is perpendicular to the line of intersection of planes and

Determine whether the line of parametric equations intersects the plane with equation If it does intersect, find the point of intersection.

The line intersects the plane at point

Determine whether the line of parametric equations intersects the plane with equation If it does intersect, find the point of intersection.

Find the distance from point to the plane of equation

Find the distance from point to the plane of equation

For the following exercises, the equations of two planes are given.

- Determine whether the planes are parallel, orthogonal, or neither.
- If the planes are neither parallel nor orthogonal, then find the measure of the angle between the planes. Express the answer in degrees rounded to the nearest integer.

**[T]**

a. The planes are neither parallel nor orthogonal; b.

a. The planes are parallel.

**[T]**

Show that the lines of equations and are skew, and find the distance between them.

Show that the lines of equations and are skew, and find the distance between them.

Consider point and the plane of equation

- Find the radius of the sphere with center tangent to the given plane.
- Find point
*P*of tangency.

a. b.

Consider the plane of equation

- Find the equation of the sphere with center at the origin that is tangent to the given plane.
- Find parametric equations of the line passing through the origin and the point of tangency.

Two children are playing with a ball. The girl throws the ball to the boy. The ball travels in

the air, curves ft to the right, and falls ft away from the girl (see the following figure). If the plane that contains the trajectory of the ball is perpendicular to the ground, find its equation.

**[T]** John allocates dollars to consume monthly three goods of prices In this context, the budget equation is defined as where and represent the number of items bought from each of the goods. The budget set is given by and the budget plane is the part of the plane of equation for which and Consider and

- Use a CAS to graph the budget set and budget plane.
- For find the new budget equation and graph the budget set in the same system of coordinates.

**[T]** Consider the position vector of a particle at time where the components of **r** are expressed in centimeters and time is measured in seconds. Let be the position vector of the particle after sec.

- Determine the velocity vector of the particle after sec.
- Find the scalar equation of the plane that is perpendicular to and passes through point This plane is called the
*normal plane*to the path of the particle at point - Use a CAS to visualize the path of the particle along with the velocity vector and normal plane at point

a. b.

c.

**[T]** A solar panel is mounted on the roof of a house. The panel may be regarded as positioned at the points of coordinates (in meters) and (see the following figure).

- Find the general form of the equation of the plane that contains the solar panel by using points and show that its normal vector is equivalent to
- Find parametric equations of line that passes through the center of the solar panel and has direction vector which points toward the position of the Sun at a particular time of day.
- Find symmetric equations of line that passes through the center of the solar panel and is perpendicular to it.
- Determine the angle of elevation of the Sun above the solar panel by using the angle between lines and

### Glossary

- direction vector
- a vector parallel to a line that is used to describe the direction, or orientation, of the line in space

- general form of the equation of a plane
- an equation in the form where is a normal vector of the plane, is a point on the plane, and

- normal vector
- a vector perpendicular to a plane

- parametric equations of a line
- the set of equations and describing the line with direction vector passing through point

- scalar equation of a plane
- the equation used to describe a plane containing point with normal vector or its alternate form where

- skew lines
- two lines that are not parallel but do not intersect

- symmetric equations of line
- the equations describing the line with direction vector passing through point

- vector equation of a line
- the equation used to describe a line with direction vector passing through point where is the position vector of point

- vector equation of a plane
- the equation where is a given point in the plane, is any point in the plane, and is a normal vector of the plane