Orlando is building a coffee table for his mother. He has the round surface cut and sanded and is ready to attach the legs. As he tries a few different heights, he notices that unless the spot on the floor is really flat, and he is extremely careful to make the legs *exactly* the same length, the table is unsteady and wobbles annoyingly.

How can Orlando ensure that the table remains stable, even if the floor is not entirely flat where his mom decides to put it?

### Planes in Space

Vectors can be used to identify directions in space and also the orientation of a plane by identifying the direction perpendicular to that plane. In this section we will look at that calculation in reverse. Rather than determining the normal vector to a plane using two vectors which lie in that plane, we will be using the normal vector to determine the equation for the plane itself. We will also use the normal vectors to determine the intersection angle between any pair of planes.

#### Intercept Form

The diagram below shows a plane that crosses all three coordinate axes. Points A, B, and C are the locations where the plane crosses each of the coordinate axes, called **intercepts**. Their locations are given by A = (a, 0, 0), B = (0, b, 0), and C = (0, 0, c). The line segments **AB**, **BC**, and **CA** all lie in the plane. Furthermore, segment **AB** is a portion of the line of intersection between this plane and the x-y axis; segment **BC** is a portion of the line of intersection between this plane and the y-z axis; and segment **CA** is a portion of the line of intersection between this plane and the z-x axis.

The **intercept form** of the equation for a plane is given by

\begin{align*}1 = \frac{x}{a} + \frac{y}{b} + \frac{z}{c}\end{align*}

#### Using a Normal Vector

Another way to specify a plane is to know two vectors within the plane. Recall that if two vectors lie in the same plane, the normal to that plane can be found using the cross product of the two vectors. Sometimes we are given the equations of the vectors themselves. Sometimes, however, we are only given a set of points that lie on the plane. If we know three points that lie on the plane, we can use the method developed when looking at cross products to find equations for the vectors between those points and then use those vectors to identify the plane. We only need three points to accomplish this task.

Since the normal to the plane is, by definition, perpendicular to all possible vectors within a plane and since the dot product of two vectors is equal to zero for any two perpendicular vectors, we can define a plane in terms of the dot product of the normal vector with any vector, \begin{align*}\overrightarrow{v}\end{align*}

\begin{align*}\overrightarrow{n} \times \overrightarrow{v} = 0\end{align*}

Which we can also write as

\begin{align*}\left \langle n_x, n_y, n_z \right \rangle \times \left \langle (x - x_0), (y - y_0), (z - z_0) \right \rangle = 0\end{align*}

If we compute the dot product, we obtain another equation which specifies the plane in terms of the normal vector and two points on the plane, (*x*, *y*, *z*) and (*x*_{0}, *y*_{0}, *z*_{0}).

*n** _{x}* (

*x*-

*x*

_{0}) +

*n*

*(*

_{y}*y*-

*y*

_{0}) +

*n*

*(*

_{z}*z*-

*z*

_{0}) = 0

This equation is frequently written as

*n*_{x}*x* + *n*_{y}*y* + *n*_{z}*z* + *d* = 0

Where

*d* = -*n*_{x}*x** _{o}* - n

_{y}*y*

*-*

_{o}*n*

_{z}*z*

_{o}and the intercepts of the plane with the x, y, and z axes are given by:

\begin{align*}a = -\frac{d}{n_x}, \ b = -\frac{d}{n_y},\end{align*}

If you only have the normal vector and one point on the plane, first determine the vector projection of the position vector of that point onto the normal vector using the dot product. Then you will have the locations of two points on the plane and can use the normal and two points method described above.

### Examples

#### Example 1

Earlier, you were given a problem about Orlando, who is making a table for his mother.

How can he ensure that the table remains stable, even if the floor is not entirely flat where his mom decides to put it?

Recall from the lesson that any plane can be defined by three points. If Orlando makes the table with only three legs, then the endpoints of each leg will define a stable plane for the table to sit on. Even if the plane defined by the leg endpoints is not exactly parallel to the average plane of the floor across the entire room, the table itself will remain stable.

#### Example 2

Find the intercepts of the plane given by the equation 3*x* + 5*y* - 2*z* - 4 = 0.

Rewrite the equation of the plane in the format of the intercept form of the plane equation.

3*x* + 5*y* - 2*z* = 4

\begin{align*}1 = \frac{3}{4} x + \frac{5}{4} y + \frac{-2}{4}z\end{align*}

\begin{align*}a = \frac{4}{3}, \ b = \frac{4}{5}\end{align*}

\begin{align*}\left (\frac{4}{3}, 0, 0 \right ), \ \left (0, \frac {4}{5}, 0 \right )\end{align*}

#### Example 3

A plane has intersections at (12, 0, 0), (0, 6, 0), and (0, 0, 4). Write the equation of the plane.

\begin{align*}1 = \frac{x}{a} + \frac{y}{b} + \frac{z}{c} = \frac{x}{12} + \frac{y}{6} + \frac{z}{4}\end{align*}

#### Example 4

Use the equation 3*x* + 5*y* - 2*z* - 4 = 0 to determine the normal unit-vector to that plane.

Comparing this equation to *n*_{x}*x* + *n*_{y}*y* + *n*_{z}*z* + *d* = 0, we can see that \begin{align*}\overrightarrow{n} = \left \langle 3, 5, -2\right \rangle\end{align*}

\begin{align*}|\overrightarrow{n}| = \sqrt{n_x^2 + n_y^2 + n_z^2} = \sqrt{3^2 + 5^2 + (-2)^2} = \sqrt{38}\end{align*}

\begin{align*}\hat{n} = \left \langle \frac{3}{\sqrt{38}}, \frac{5}{\sqrt{38}}, \frac{-2}{\sqrt{38}} \right \rangle\end{align*}

#### Example 5

The three points P = (3, 7, 2), Q = (1, 4, 3), and R = (2, 3, 4) define a plane. Determine the equation of the plane.

First find the vectors between two pairs of the points.

\begin{align*}\overrightarrow{PQ} = \left \langle (Q_x - P_x), (Q_y - P_y), (Q_z - P_z) \right \rangle = \left \langle (1 - 3), (4 - 7), (3 - 2) \right \rangle =\end{align*}

\begin{align*}\overrightarrow{PR} = \left \langle (R_x - P_x), (R_y - P_y), (R_z - P_z) \right \rangle = \left \langle (2 - 3), (3 - 7), (4 - 2) \right \rangle =\end{align*}

The cross product of these two vectors is normal to the plane.

\begin{align*}\overrightarrow{PQ} \times \overrightarrow{PR} = \left \langle (PQ_yPR_z - PQ_zPR_y), (PQ_zPR_x - PQ_xPR_z), (PQ_xPR_y - PQ_yPR_x)\right \rangle\end{align*}

\begin{align*}\overrightarrow{PQ} \times \overrightarrow{PR} = \left \langle [(-3 \cdot 2) - (1 \cdot -4)], [(1 \cdot -1) - (-2 \cdot 2)], [(-2 \cdot -4) - (-1 \cdot -3)] \right \rangle\end{align*}

\begin{align*}\overrightarrow{n} = \overrightarrow{PQ} \times \overrightarrow{PR} = \left \langle [(-6) - (-4)], [(-1) - (-4)], [(8) - (3)] \right \rangle = \left \langle -2, 3, 5 \right \rangle\end{align*}

This normal vector and one of the points will give an equation for the plane.

*n** _{x}* (

*x*-

*P*

*) +*

_{x}*n*

*(*

_{y}*y*-

*P*

*) +*

_{y}*n*

*(*

_{z}*z*-

*P*

*) = 0*

_{z}-2(*x* - 3) + 3(*y* - 7) + 5(*z* - 2) = 0

-2*x* + 3*y* + 5*z* + (6 - 21 - 10) = 0

-2*x* + 3*y* + 5*z* - 25 = 0

#### Example 6

Rewrite the equation of the plane 7*x* + 3*y* + *z* + 12 = 0 in intercept form.

The equation \begin{align*}1 = \frac{x}{a} + \frac{y}{b} + \frac{z}{c}\end{align*}*x* + 3*y* + *z* + 12 = 0 into the form \begin{align*}1 = \frac{x}{a} + \frac{y}{b} + \frac{z}{c}\end{align*}

7*x* + 3*y* + *z* = -12

\begin{align*}\frac{7}{-12}x + \frac{3}{-12}y + \frac{1}{-12}z = 1\end{align*}

Therefore, \begin{align*}a = \frac{-12}{7}, \ b = \frac{-12}{3} = -4\end{align*}

#### Example 7

Determine the equation for the unit vector which is perpendicular to the plane, 7*x* + 3*y* + *z* + 12 = 0.

Comparing this equation to \begin{align*}n_xx + n_yy + n_zz + d = 0\end{align*}

\begin{align*}\hat{n} = \frac{\overrightarrow{n}}{|\overrightarrow{n}|} = \frac{\left \langle n_x, n_y, n_z \right \rangle}{\sqrt{n_x^2 + n_y^2 + n_z^2}} = \frac{\left \langle 7, 3, 1 \right \rangle}{\sqrt{(7)^2 + (3)^2 + (1)^2}} = \frac{\left \langle 7, 3, 1 \right \rangle}{\sqrt{49 + 9 + 1}}\end{align*}

#### \begin{align*}= \frac{\left \langle 7, 3, 1 \right \rangle}{\sqrt{59}} = \left \langle \frac{7}{\sqrt{59}}, \frac{3}{\sqrt{59}}, \frac{1}{\sqrt{59}} \right \rangle\end{align*}

#### Example 8

A plane is defined by three points having position *vectors* \begin{align*}\overrightarrow{r_1} = \left \langle 1, 0, -1 \right \rangle, \ \overrightarrow{r_2} = \left \langle 2, 4, 6 \right \rangle\end{align*}, and \begin{align*}\overrightarrow{r_3} = \left \langle -3, 7, 5 \right \rangle\end{align*}. Determine the components of the unit vector which is perpendicular to the plane passing through those points.

The cross-product determines the direction perpendicular to a pair of vectors. Therefore we can use these three points to define two vectors in the same plane. The vector from point 1 to point 2 is given by subtracting vector 2 from vector 1:

\begin{align*}\overrightarrow{r_{1-2}} = \overrightarrow{r_1} - \overrightarrow{r_2} = \left \langle 1, 0, -1\right \rangle - \left \langle 2, 4, 6 \right \rangle = \left \langle 1 - 2, 0 - 4, -1 - 6 \right \rangle = \left \langle -1, -4, -7 \right \rangle\end{align*}

Likewise, the vector from point 1 to point 3 is given by subtracting vector 3 from vector 1:

\begin{align*}\overrightarrow{r_{1-3}} = \overrightarrow{r_1} - \overrightarrow{r_3} = \left \langle 1, 0, -1\right \rangle - \left \langle -3, 7, 5 \right \rangle = \left \langle 1 - (-3), 0 - 7, -1 - 5 \right \rangle =\end{align*} \begin{align*}\left \langle 4, -7, -6 \right \rangle\end{align*}

Now we can use the cross-product of the two vectors in the plane to determine a vector which is perpendicular to that plane,

\begin{align*}\overrightarrow{n} = \overrightarrow{r_{1-2}} \times \overrightarrow{r_{1-3}} = \left \langle (r_{1-2y}r_{1-3z} - r_{1-2z}r_{1-3y}), (r_{1-2z}r_{1-3x} - r_{1-2x}r_{1-3z}), (r_{1-2x}r_{1-3y} - r_{1-2y}r_{1-3x}) \right \rangle\end{align*}

\begin{align*}\overrightarrow{n} = \overrightarrow{r_{1-2}} \times \overrightarrow{r_{1-3}} = \left \langle ((-4)(-6) - (-7)(-7)), ((-7)(4) - (-1)(-6)), ((-1)(-7) - (-4)(4)) \right \rangle\end{align*}

\begin{align*}\overrightarrow{n} = \overrightarrow{r_{1-2}} \times \overrightarrow{r_{1-3}} = \left \langle ((24) - (49), ((-28) - (6)), ((7) - (-16)) \right \rangle = \left \langle -25, -34, 23 \right \rangle\end{align*}

Now use the definition of the unit vector to complete the problem.

\begin{align*}\hat{n} = \frac{\overrightarrow{n}}{|\overrightarrow{n}|} = \frac{\left \langle n_x, n_y, n_z \right \rangle}{\sqrt{n_x^2 + n_y^2 + n_z^2}} = \frac{\left \langle -25, -34, 23 \right \rangle}{\sqrt{(-25)^2 + (-34)^2 + (23)^2}}\end{align*}

\begin{align*}= \frac{\left \langle -25, -34, 23 \right \rangle}{\sqrt{625 + 1156 + 529}} = \frac{\left \langle -25, -34, 23 \right \rangle}{\sqrt{2310}} \end{align*}

\begin{align*}\hat{n} = \frac{\overrightarrow{n}}{|\overrightarrow{n}|} = \frac{\left \langle {-25, -34, 23}\right \rangle}{48.06} = \left \langle -0.5202, -0.7074, 0.4785 \right \rangle\end{align*}

#### Example 9

Determine the components of the unit vector which is perpendicular to the plane 12*x* + 23*y* + 14*z* - 5 = 0.

Comparing this equation to \begin{align*}n_xx + n_yy + n_zz + d = 0\end{align*}, we can see that \begin{align*}\overrightarrow{n} = \left \langle 12, 23, 14 \right \rangle\end{align*}.

Now we can use the definition of the unit vector to complete the problem.

\begin{align*}\hat{n} = \frac{\overrightarrow{n}}{|\overrightarrow{n}|} = \frac{\left \langle n_x, n_y, n_z \right \rangle}{\sqrt{n_x^2 + n_y^2 + n_z^2}} = \frac{\left \langle 12, 23, 14 \right \rangle}{\sqrt{12^2 + 23^2 + 14^2}} = \frac{\left \langle 12, 23, 14 \right \rangle}{\sqrt{869}}\end{align*}

\begin{align*}= \frac{\left \langle 12, 23, 14 \right \rangle}{29.5} = \left \langle \frac{12}{29.5}, \frac{23}{29.5}, \frac{14}{29.5} \right \rangle\end{align*}

### Review

Given the following intersections, write the equation of the plane.

- \begin{align*}(7, 0, 0), (0, 3, 0)\end{align*} and \begin{align*}(0, 0, 19)\end{align*}
- \begin{align*}(2, 0, 0), (0, 8, 0)\end{align*} and \begin{align*}(0, 0, 5)\end{align*}
- \begin{align*}(13, 0, 0), (0, 21, 0)\end{align*} and \begin{align*}(0, 0, 17)\end{align*}
- \begin{align*}(5, 0, 0), (0, 1, 0)\end{align*} and \begin{align*}(0, 0, 2)\end{align*}
- \begin{align*}(27, 0, 0), (0, 12, 0)\end{align*} and \begin{align*}(0, 0, 18)\end{align*}

Find the intercepts of the plane given the following equations.

- \begin{align*}3x + 2y + z - 6 = 0\end{align*}
- \begin{align*}1x - 7y - z + 10 = 0\end{align*}
- \begin{align*}-2x + 9y + 4z - 1 = 0\end{align*}
- \begin{align*}6x - 11y + 2z + 3 = 0\end{align*}
- \begin{align*}-2x + 5y + 5z + 6 = 0\end{align*}

Use the given equations to determine the normal unit-vector to that plane.

- \begin{align*}7x + 5y - 1z - 10 = 0\end{align*}
- \begin{align*}4x - 13y + 5z - 3 = 0\end{align*}
- \begin{align*}-8x + 7y + 2z + 5 = 0\end{align*}
- \begin{align*}10x + 3y - z - 2 = 0\end{align*}
- \begin{align*}-1x - 2y + 7z + 16 = 0\end{align*}

Determine the equation of the planes below using the three points given.

- \begin{align*}P = (3, 6, 9), Q = (9, 6, 3)\end{align*} and \begin{align*}R = (6, -9, 9)\end{align*}
- \begin{align*}P = (1, -7, 2), Q = (4, 2, 9)\end{align*} and \begin{align*}R = (3, -5, 1)\end{align*}
- \begin{align*}P = (3, 8, 10), Q = (-2, 5, 8)\end{align*} and \begin{align*}R = (7, 4, 8)\end{align*}
- \begin{align*}P = (9, -1, 4), Q = (6, 2, -8)\end{align*} and \begin{align*}R = (12 , 9, 10)\end{align*}
- \begin{align*}P = (5, 8,-9), Q = ( -5, 3, 9)\end{align*} and \begin{align*}R = (10, 4, -6)\end{align*}

### Review (Answers)

To see the Review answers, open this PDF file and look for section 5.7.