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Planes in R3\colorOne{\reals^3}

A plane is an infinite flat surface in 3D space.

Planes can be described using various pieces of information:
  • A point on the plane and a normal vector (orthogonal to the plane)
  • Two lines on the plane
  • Three points on the plane
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Point-Normal Form

A plane in R3\mathbb{R}^3 can be described in point-normal form (or vector form) using one point on the plane, and one normal vector.

Given a known point on the plane P(x1,y1,z1)P(x_1,y_1,z_1) and a normal vector n⃗=(a,b,c)\vec{n}=(a,b,c):
n⃗⋅(x⃗−P⃗)=0\boxed{\quad \vec{n}\cdot (\vec x - \vec P)=0 \quad}
  ⟹  a(x−x1)+b(y−y1)+c(z−z1)=0\quad\implies\quad a(x-x_1)+b(y-y_1)+c(z-z_1)=0

Standard Form

Simplifying the point-normal form gives the standard form (or scalar form) of a plane in R3\reals^3, using d=n⃗⋅P⃗d=\vec n \cdot \vec P:
ax+by+cz=d\boxed{\quad ax+by+cz=d \quad}

Geometrically

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Parametric Form

Given a point on the plane PP, and two non-collinear vectors in the plane d⃗1, d⃗2\vec d_1 , \ \vec d_2, we may introduce two parameters, s ,t∈Rs\ ,t \in \reals:
X⃗=P⃗+s d⃗1+t d⃗2\boxed{\quad \vec X = \vec P + s\ \vec d_1 + t\ \vec d_2 \quad}
Geometrically


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Given Two Lines on the Plane

Steps
  1. Find the normal vector n⃗=d1⃗×d2⃗\vec n = \vec{d_1} \times \vec{d_2} where d1⃗, d2⃗\vec{d_1}, \ \vec{d_2} are the direction vectors of the lines.
  2. Find any point PP on one of the lines (this is a known point on the plane).
  3. Use n⃗\vec n and PP to write the equation of the plane in point-normal form or standard form, or use PP and d⃗1, d⃗2\vec d_1,\ \vec d_2 to write the plane in parametric form.

Given Three Points on the Plane

Knowing three points P,Q,RP, Q, R that lie in the plane allows us to find two lines on the plane, so we can proceed as above.
Steps
  1. Find direction vectors in the plane: for example, let d1⃗=PQ→\vec{d_1} = \overrightarrow{PQ} and d2⃗=PR→\vec{d_2} = \overrightarrow{PR}.
  2. Find the normal vector n⃗=d1⃗×d2⃗\vec n = \vec{d_1} \times \vec{d_2} where d1⃗, d2⃗\vec{d_1}, \ \vec{d_2} are the direction vectors of the lines.
  3. Use n⃗\vec n and PP to write the equation of the plane in point-normal form or standard form, or use PP and d⃗1, d⃗2\vec d_1,\ \vec d_2 to write the plane in parametric form.


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Example: Planes in R3\colorOne{\reals^3}

Find the equation of the plane with the normal vector ⟨3,1,−1⟩\lang 3,1,−1 \rang that passes through point P(0,2,1)P(0,2,1).

Point-Normal Form

n⃗⋅(x⃗−P⃗)=0\vec n \cdot (\vec x - \vec P) = 0

n⃗=[31−1]\vec n= \begin{bmatrix} 3\\ 1\\ -1\\ \end{bmatrix}
Substituting:
[31−1]⋅([xyz]−[021])=0\boxed{ \begin{bmatrix} 3\\ 1\\ -1\\ \end{bmatrix} \cdot \left( \begin{bmatrix} x\\ y\\ z\\ \end{bmatrix} − \begin{bmatrix} 0\\ 2\\ 1\\ \end{bmatrix} \right) = 0 }
We could simplify to obtain a scalar equation:
3(x−0)+(y−2)−(z−1)=0 3(x-0) + (y-2) - (z-1) = 0

Standard form

ax+by+cz=dax+by+cz=d where n⃗=⟨a,b,c⟩\vec n = \lang a,b,c \rang and d=n⃗⋅P⃗d=\vec{n}\cdot\vec{P}

n⃗=[abc]=[31−1]\vec n = \begin{bmatrix} a\\ b\\ c\\ \end{bmatrix} = \begin{bmatrix} 3\\ 1\\ -1\\ \end{bmatrix}
d=n⃗⋅P⃗=[31−1]⋅[021]=(3)(0)+(1)(2)+(−1)(1)=1\begin{aligned} d &= \vec n \cdot \vec P\\[0.3em] &= \begin{bmatrix} 3\\ 1\\ -1\\ \end{bmatrix} \cdot \begin{bmatrix} 0\\ 2\\ 1\\ \end{bmatrix}\\[1.7em] &=(3)(0)+(1)(2)+(−1)(1)\\[0.3em] &=1 \end{aligned}

Now we substitute into the standard form formula using the normal vector where a=3a=3, b=1b=1, c=−1c=-1:
3x+y−z=1\boxed{3x+y−z=1}
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Example: Planes in R3\colorOne{\reals^3}

Find the parametric, point-normal, and standard form equations of the plane containing the points A(−6,−3,−1)A(-6,-3,-1) , B(0,0,−5)B(0,0,-5), and C(−9,−3,0)C(-9,-3,0).

We start by finding two direction vectors:
d1⃗=AB→=[00−5]−[−6−3−1]=[63−4]\begin{aligned} \vec{d_1} &= \overrightarrow{AB}\\ &= \begin{bmatrix} 0\\ 0\\ -5\\ \end{bmatrix} -\begin{bmatrix} -6\\ -3\\ -1\\ \end{bmatrix}\\[1.5em] &= \begin{bmatrix} 6\\ 3\\ -4\\ \end{bmatrix} \end{aligned} d2⃗=BC→=[−9−30]−[00−5]=[−9−35]\begin{aligned} \vec{d_2} &= \overrightarrow{BC}\\ &= \begin{bmatrix} -9\\ -3\\ 0\\ \end{bmatrix} -\begin{bmatrix} 0\\ 0\\ -5\\ \end{bmatrix}\\[1.5em] &= \begin{bmatrix} -9\\ -3\\ 5\\ \end{bmatrix} \end{aligned}

We can now write the plane in parametric form with point B(0,0,−5)\colorOne{B(0,0,-5)} and vectors d⃗1=⟨6,3,−4⟩,   d⃗2=⟨−9,−3,5⟩\colorThree{\vec d_1 = \lang 6,3,-4 \rang},\ \ \ \colorFive{\vec d_2 = \lang -9,-3,5 \rang}:
⟨x,y,z⟩=⟨0,0,−5⟩+s⟨6,3,−4⟩+t⟨−9,−3,5⟩\boxed{ \lang x,y,z \rang = \colorOne{\lang 0,0,-5 \rang} + s \colorThree{\lang 6,3,-4 \rang} + t \colorFive{\lang -9,-3,5 \rang} }

To write the plane in point-normal or standard form, we need to find a normal vector so we find the cross product of the direction vectors:

n⃗=d1⃗×d2⃗=[63−4]×[−9−35]=[(3)(5)−(−4)(−3)(−4)(−9)−(6)(5)(6)(−3)−(3)(−9)]=[369]\begin{aligned} \vec{n} &= \vec{d_1} \times \vec{d_2}\\ &= \begin{bmatrix} 6\\ 3\\ -4\\ \end{bmatrix} \times \begin{bmatrix} -9\\ -3\\ 5\\ \end{bmatrix}\\[1.5em] &= \begin{bmatrix} (3)(5)-(-4)(-3)\\[0.5em] (-4)(-9)-(6)(5)\\[0.5em] (6)(-3)-(3)(-9)\\[0.5em] \end{bmatrix}\\[1.5em] &= \begin{bmatrix} 3\\ 6\\ 9\\ \end{bmatrix} \end{aligned}

Using n⃗=⟨3,6,9⟩\colorFour{\vec n = \lang 3,6,9 \rang} and point B(0,0,−5)\colorOne{B(0,0,-5)} we can write:

⟨3,6,9⟩⋅(⟨x,y,z⟩−⟨0,0,−5⟩)=0[point-normal form]⟨3,6,9⟩⋅⟨x−0, y−0, z−(−5)⟩=03(x−0)+6(y−0)+9(z−(−5))=03x+6y+9(z+5)=03[ x+2y+3(z+5) ]=0x+2y+3(z+5)=0x+2y+3z=−15[standard form]\begin{array}{rlr} \colorFour{\lang 3,6,9 \rang} \cdot (\lang x,y,z \rang - \colorOne{\lang 0,0,-5 \rang}) &= 0 \quad \text{[point-normal form]}\\[0.5em] \colorFour{\lang 3,6,9 \rang} \cdot \lang x-\colorOne{0},\ y-\colorOne{0},\ z-\colorOne{(-5)} \rang &= 0\\[0.5em] \colorFour{3}(x-\colorOne{0}) +\colorFour{6}(y-\colorOne{0}) +\colorFour{9}(z-\colorOne{(-5)}) &= 0\\[0.5em] 3x + 6y + 9(z+5) &= 0\\[0.5em] 3\big[\ x + 2y + 3(z+5)\ \big] &= 0\\[0.5em] x + 2y + 3(z+5)&= 0\\[0.5em] x+2y+3z &= -15 \quad \text{[standard form]}\\[0.5em] \end{array}

Alternatively
We could have jumped straight to standard form using the formula:
n⃗⋅x⃗=n⃗⋅B⃗3x+6y+9z=[369]⋅[00−5]3x+6y+9z=−45(multiply by 13)x+2y+3z=−15\begin{aligned} \vec n \cdot \vec x &= \vec n \cdot \vec B\\[0.5em] \colorFour{3}x +\colorFour{6}y +\colorFour{9}z &= \colorFour{ \begin{bmatrix} 3\\ 6\\ 9\\ \end{bmatrix}} \cdot \colorOne{ \begin{bmatrix} 0\\ 0\\ -5\\ \end{bmatrix}} \\[1.5em] 3x +6y +9z &= -45 \qquad \text{\small (multiply by $\dfrac{1}{3}$)}\\[0.5em] \colorTwo{x +2y +3z} &\colorTwo{= -15} \end{aligned}

Practice: Parallel Planes in R3\colorOne{\reals^3}

Find the values of aa and bb such that the following planes are parallel:
Π1:  3x+2y+z=8Π_1:\ \ 3x+2y+z=8
Π2:  ax+6y−bz=9Π_2:\ \ ax+6y−bz=9

Practice: Planes in R3\colorOne{\reals^3}

Find the standard form equation of the plane that contains the points A(1,−1,0),  B(1,1,6), C(2,0,3)A\left(1,-1,0\right),\ \ B\left(1,1,6\right),\ C\left(2,0,3\right).

Practice: Planes in R3\colorOne{\reals^3} (Tricky)

Consider the points A(−1,3,2)A(-1,3,2), B(−3,5,0)B(-3,5,0), and C(xC,yC,zC)C(x_C,y_C,z_C), and suppose we know the vector AC→=⟨3,−4,−5⟩\overrightarrow{AC}=\lang 3,-4,-5\rang.

Find the equation of the plane containing point CC, and perpendicular to the line passing through points AA and BB.
Extra Practice