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Vectors in space · Metric geometry

Distances in Space

Before you start, make sure you know: Distances in the plane (High School Year 2 (semesters 3 and 4))
Before you start, make sure you know: Dot product (High School Year 2 (semesters 3 and 4))

A sounding rocket passes near a tracking station: what is the minimum distance? Two beams in a pavilion almost touch: is a safety connector needed between them? All distances in space come down to a few formulas, each tied to a tool you already know: the magnitude of a vector, the dot product, the cross product, or the scalar triple product. Here we bring them together so you can choose the right one depending on what you are given.

Distance between two points

Segment in space between two points A and B: the distance is the magnitude of the vector joining them. A B d(A, B)
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In a 3D video game development environment, two objects are located at points $A(2, -3, 1)$ and $B(6, 1, 1)$, where units are in meters. Calculate the distance…

In a 3D video game development environment, two objects are located at points $A(2, -3, 1)$ and $B(6, 1, 1)$, where units are in meters. Calculate the distance separating both objects to program a collision if it is less than 5 meters.

X Y Z 2 6 −3 1 1 d=? A(2, −3, 1) B(6, 1, 1) if (d < 5.0 ) { triggerCollision (); }
Solve
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Calculate the coordinates of the points on the x-axis (axis $OX$) whose distance to the point $P(3, 2, 2)$ is exactly $\sqrt{8}$ units. Explain geometrically why you…

Calculate the coordinates of the points on the x-axis (axis $OX$) whose distance to the point $P(3, 2, 2)$ is exactly $\sqrt{8}$ units. Explain geometrically why you obtain a unique solution.

Solve

Distance from a point to a plane

Perpendicular segment from an exterior point P to a plane to measure its distance. P d
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A cell phone transmitter antenna is located at point $P(1, 2, -3)$. It is known that the signal is optimally intercepted by a flat reflecting panel whose equation is…

A cell phone transmitter antenna is located at point $P(1, 2, -3)$. It is known that the signal is optimally intercepted by a flat reflecting panel whose equation is $\pi: 3x - 4z + 2 = 0$. Determine the minimum distance the wave must travel from the antenna to the panel.

Solve
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A laser depth sensor is located at point $S(5, -7, 4)$. The laser points directly towards the side wall, represented by the coordinate plane $XZ$. How far is the sensor…

A laser depth sensor is located at point $S(5, -7, 4)$. The laser points directly towards the side wall, represented by the coordinate plane $XZ$. How far is the sensor from this wall?

Solve
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Find the coordinates of the points on the line $r \equiv x = y = z$ that are at a distance of $2\sqrt{3}$ units from the plane $\pi \equiv x + 2y - 2z + 1 = 0$.

Find the coordinates of the points on the line $r \equiv x = y = z$ that are at a distance of $2\sqrt{3}$ units from the plane $\pi \equiv x + 2y - 2z + 1 = 0$.

Solve

Distance from a point to a line

Perpendicular distance from a point P to a line r, seen as the height of the parallelogram formed by AP and the direction vector. r A P d
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Consider the point $A(1, 5, -2)$ and the line $r$ defined by its parametric equations: $$r: \begin{cases} x = 2 + t \\ y = -1 \\ z = 3 \end{cases}$$ Find the distance…

Consider the point $A(1, 5, -2)$ and the line $r$ defined by its parametric equations:

$$r: \begin{cases} x = 2 + t \\ y = -1 \\ z = 3 \end{cases}$$

Find the distance from point $A$ to line $r$ using the vector product formula.

Solve
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A sounding rocket follows a linear trajectory defined by the line $r \equiv \begin{cases} x = 1 + \lambda \\ y = 2 - \lambda \\ z = 3 + 2\lambda \end{cases}$. The rocket…

A sounding rocket follows a linear trajectory defined by the line $r \equiv \begin{cases} x = 1 + \lambda \\ y = 2 - \lambda \\ z = 3 + 2\lambda \end{cases}$. The rocket must send a signal to the tracking station located at point $P(5, -1, 4)$. Calculate the minimum distance at which the rocket will pass the tracking station during its flight.

r Aᵣ(1, 2, 3) vᵣ(1, −1, 2) P(5, −1, 4) AᵣP Pᵣ d
Solve
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Consider the cube with edge length 4 units, having one vertex at the origin $(0,0,0)$ and its edges lying along the positive coordinate axes. Calculate the distance from…

Consider the cube with edge length 4 units, having one vertex at the origin $(0,0,0)$ and its edges lying along the positive coordinate axes. Calculate the distance from vertex $V(4, 4, 4)$ to the cube's diagonal joining points $(4, 0, 0)$ and $(0, 4, 0)$.

X Y Z V(4,4,4) A(4,0,0) B(0,4,0) M d O(0,0,0)
Solve

Distance between two lines

Two skew lines in space, r and s, not parallel, and the segment d perpendicular to both that measures their minimum distance. r s d
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The support structures of a pavilion are being designed. Two of the main beams are modelled by the lines $r \equiv \frac{x-1}{2} = \frac{y}{1} = \frac{z+1}{-1}$ and…

The support structures of a pavilion are being designed. Two of the main beams are modelled by the lines $r \equiv \frac{x-1}{2} = \frac{y}{1} = \frac{z+1}{-1}$ and $s \equiv \frac{x}{1} = \frac{y-2}{2} = \frac{z}{3}$. Determine the minimum distance separating both beams to check whether it is necessary to install a safety connector between them.

Solve
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Two communications satellites follow linear paths in space (measured in hundreds of kilometers) represented by the lines $r$ and $s$:…

Two communications satellites follow linear paths in space (measured in hundreds of kilometers) represented by the lines $r$ and $s$:

$r: \begin{cases} x = 1 + \lambda \\ y = 2 - \lambda \\ z = \lambda \end{cases}$ and $s: \frac{x-3}{1} = \frac{y-1}{2} = \frac{z-k}{1}$

Find:

  1. Determine the values of the parameter $k$ so that the minimum distance between the two paths is exactly $\sqrt{6}$ units (600 km).

  2. For each value of $k$ obtained in the previous section, calculate the points on each path where this minimum distance is reached (points of the common perpendicular).

πₛ r' s r d = √6 Pᵣ Pₛ
Solve
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An architect designs an avant-garde structure made up of two tensioned cables. In a Cartesian coordinate system, the first cable coincides with the line $r$ passing…

An architect designs an avant-garde structure made up of two tensioned cables. In a Cartesian coordinate system, the first cable coincides with the line $r$ passing through $A(1, 1, 1)$ and $B(2, 3, 0)$. The second cable follows the direction of vector $\vec{u} = (1, 0, 1)$ and passes through point $C(0, 2, 2)$.

Calculate:

  1. The minimum distance between the two cables.

  2. The equation of the line representing a metallic reinforcement joining both cables along the shortest possible path (common perpendicular).

X Y Z A(1,1,1) B(2,3,0) r C(0,2,2) u s d
Solve

Distance between parallel planes and between a line and a plane

Common perpendicular segment that measures the constant distance between two parallel planes. π₁ π₂ d
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A metal rail has been installed for an industrial robot following the trajectory of the line: $$r: \frac{x-1}{2} = \frac{y+3}{0} = \frac{z-2}{1}$$ If the factory floor…

A metal rail has been installed for an industrial robot following the trajectory of the line:

$$r: \frac{x-1}{2} = \frac{y+3}{0} = \frac{z-2}{1}$$

If the factory floor is the plane $\pi: x - 2z + 5 = 0$, first check that the rail is parallel to the floor and then calculate the safety distance of the rail from the floor.

π r P P' d
Solve
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In an archaeological excavation, it has been determined that the original floor of a room lies on the plane $\pi \equiv 2x - y + 2z - 6 = 0$. A set of sensors moves…

In an archaeological excavation, it has been determined that the original floor of a room lies on the plane $\pi \equiv 2x - y + 2z - 6 = 0$. A set of sensors moves along a metal track represented by the line $r \equiv \begin{cases} x = \mu \\ y = 4\mu - 1 \\ z = \mu + 1 \end{cases}$.

  1. a) Verify that the track is parallel to the room's floor.
  2. b) Calculate the constant distance between the sensors and the floor.
Metal sensor track parallel to the archaeological floor and distance between themzyxπRoom floor: plane π ≡ 2x − y + 2z − 6 = 0n⃗πrtrackv⃗rSensor PP'd = ?
Solve
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Calculate the value of the parameter $k$ so that the distance between the parallel planes $\pi_1 \equiv 2x + 2y - z + 3 = 0$ and $\pi_2 \equiv 4x + 4y - 2z + k = 0$ is 3…

Calculate the value of the parameter $k$ so that the distance between the parallel planes $\pi_1 \equiv 2x + 2y - z + 3 = 0$ and $\pi_2 \equiv 4x + 4y - 2z + k = 0$ is 3 units. Explain why there are two possible values for $k$.

Solve

Distances and loci

With the formulas above you can solve problems where distance is a condition: points equidistant from others, tangent spheres, reflections.

  • Equidistant from two points $A$ and $B$: set $|\vec{PA}|=|\vec{PB}|$ with $P(x,y,z)$. The result is a plane: the perpendicular bisector plane of $AB$.

  • Sphere tangent to a plane: its radius is the distance from the center to the plane.

  • Given distance from a plane: write the point in parametric form, $P(\lambda)$, and solve $d(P,\pi)=k$; the absolute value gives two solutions.

  • Reflection (laser on a mirror): reflect a point across the mirror and draw the line to the other point; the shortest path is that line.

Sphere and plane with a given normal

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Determine the locus of points in space that are equidistant from points $A(1, -2, 3)$ and $B(3, 0, 1)$. What geometric figure does it represent? Calculate the distance…

Determine the locus of points in space that are equidistant from points $A(1, -2, 3)$ and $B(3, 0, 1)$. What geometric figure does it represent? Calculate the distance from the origin of coordinates to this locus.

Solve
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A laser light beam leaves point $A(1, 1, 1)$ in the direction of vector $\vec{v} = (1, 0, 2)$. We want to place a flat mirror parallel to plane $XY$ such that the…

A laser light beam leaves point $A(1, 1, 1)$ in the direction of vector $\vec{v} = (1, 0, 2)$. We want to place a flat mirror parallel to plane $XY$ such that the distance from point $A$ to the mirror is 5 units.

  1. a) Find the two possible equations of the planes containing the mirror.
  2. b) Calculate the distance from the origin to each of those planes.
Solve
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Consider a plane $\pi$ passing through the point $M(1, 0, 3)$ and perpendicular to the line $r: \frac{x-2}{1} = \frac{y+1}{2} = \frac{z}{1}$. 1. Find the equation of the…

Consider a plane $\pi$ passing through the point $M(1, 0, 3)$ and perpendicular to the line $r: \frac{x-2}{1} = \frac{y+1}{2} = \frac{z}{1}$.

  1. Find the equation of the sphere tangent to the plane $\pi$ with its center at the point $C(4, 5, 2)$.

  2. Calculate the distance from the origin of coordinates to the point of tangency between the sphere and the plane.

Solve

Closest point, reflection, and common perpendicular

Sometimes the distance is not enough: you are asked for the point where it is attained, or for the reflected point. The idea is always the same: the shortest segment is perpendicular.

Point P, its projection F onto the line r, and its reflection P' on the other side of the line, with F as the midpoint. r P P' F

If the line is given as the intersection of two planes, for example $\begin{cases}x+y=3\\2x-z=4\end{cases}$, convert it to parametric form: take $x=\lambda$ and solve for $y=3-\lambda$, $z=2\lambda-4$.

Antenna coverage. If a trajectory is $R(\lambda)$ and the antenna is at $Q$ with range $\rho$, there is signal when $|\vec{QR}|^2\le\rho^2$. This is a quadratic inequality in $\lambda$: its roots are the values of $\lambda$ where coverage begins and ends.

Common perpendicular. For two skew lines, take $R(\lambda)\in r$ and $S(\mu)\in s$ and impose $\vec{RS}\cdot\vec u=0$ and $\vec{RS}\cdot\vec v=0$. This is a system of two equations in $\lambda$ and $\mu$; its solutions give the closest points on each line, and the distance $|\vec{RS}|$ matches the scalar triple product formula.

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A logistics company uses drones to transport packages. A drone travels from point $A(1, 0, 2)$ in a straight line toward point $B(5, 4, 10)$. In the center of the…

A logistics company uses drones to transport packages. A drone travels from point $A(1, 0, 2)$ in a straight line toward point $B(5, 4, 10)$. In the center of the operating area, there is a transmitting antenna located at point $P(3, 5, 1)$.

Calculate:

  1. The exact point on the drone's path that is at the minimum distance from the antenna.

  2. If the signal from the antenna has a range of $5$ units, determine during which interval of its route (values of the parameter $\lambda$) the drone has coverage.

r=5 A(1,0,2) B(5,4,10) P(3,5,1) M dₘᵢₙ
Solve
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A laser beam starts from point $A(5, 2, 6)$ and reflects off a flat mirror represented by the coordinate plane $XY$ ($z=0$). The reflected beam must pass through point…

A laser beam starts from point $A(5, 2, 6)$ and reflects off a flat mirror represented by the coordinate plane $XY$ ($z=0$). The reflected beam must pass through point $B(1, 4, 2)$.

Find:

  1. The coordinates of the impact point $C$ of the laser on the mirror.

  2. The total distance traveled by the beam from point $A$ to point $B$. (Note: The minimum distance is obtained using the symmetric point).

Plane z=0 A (5, 2, 6) B (1, 4, 2) B' (1, 4, −2) C (x, y, 0)
Solve

Points equidistant from two planes

Two intersecting planes divide space into four regions. The points that are at the same distance from both form the bisector planes.

For a line $r$ the procedure is the one from the previous section: put $P(\lambda)$ into both distances and solve the two linear equations that come from $\pm$; each gives one point (or none, or the whole line).

If the distance to both planes is $0$, the point is on both planes at once, that is, on their line of intersection. For a line $r$ this happens when $r$ meets that common line.

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Find the coordinates of the points on the line $r: \frac{x-1}{2} = \frac{y+2}{3} = \frac{z-3}{-1}$ that are equidistant from the planes: $\pi_1: x + 2y - 2z + 1 = 0$…

Find the coordinates of the points on the line $r: \frac{x-1}{2} = \frac{y+2}{3} = \frac{z-3}{-1}$ that are equidistant from the planes:

$\pi_1: x + 2y - 2z + 1 = 0$

$\pi_2: 8x - 4y + z - 5 = 0$

Analyse whether there is any point on the line whose distance to both planes is zero and explain its geometric meaning.

Solve

Pencils of planes and the volume of a tetrahedron

A family (pencil) of planes depends on a parameter $a$, for example $\pi_a: ax+y+(a+1)z-3=0$. If all the planes share a line, that line is the intersection of any two of them.

Then, the distance from a point to that line is computed with the point-line formula, and the distance from a point to $\pi_a$ with the point-plane formula, setting it equal to the given value: this leaves an equation in $a$.

Volume of a tetrahedron. A tetrahedron $ABCD$ is one sixth of the parallelepiped formed by $\vec{AB}$, $\vec{AC}$, and $\vec{AD}$:

$$V=\frac16\left|[\vec{AB},\vec{AC},\vec{AD}]\right|$$

To find the points where a plane meets the axes, set two coordinates to zero: with $\pi: x+2y-z=4$ the $OX$ axis is met at $(4,0,0)$ and the $OY$ axis at $(0,2,0)$.

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Consider the point $P(1, 2, -1)$ and the line $r: \begin{cases} x + y = 3 \\ 2x - z = 4 \end{cases}$. 1. Calculate the symmetric point of $P$ with respect to the line…

Consider the point $P(1, 2, -1)$ and the line $r: \begin{cases} x + y = 3 \\ 2x - z = 4 \end{cases}$.

  1. Calculate the symmetric point of $P$ with respect to the line $r$. Call it $P'$.

  2. Determine the distance from $P'$ to the plane $\pi: 2x - 3y + z + 10 = 0$.

  3. What is the volume of the tetrahedron formed by the point $P$, its projection onto the line $r$, and the points of intersection of the plane $\pi$ with the axes $OX$ and $OY$?

Solve
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Given the family of planes $\pi_a: ax + (a-1)y + z + (a-2) = 0$, where $a \in \mathbb{R}$: 1. Show that all planes in the family contain the same common line $r$. Find…

Given the family of planes $\pi_a: ax + (a-1)y + z + (a-2) = 0$, where $a \in \mathbb{R}$:

  1. Show that all planes in the family contain the same common line $r$. Find the equations of this line.

  2. Calculate the distance from point $Q(2, -1, 0)$ to the line $r$.

  3. Determine the value of $a$ for which the plane $\pi_a$ is at a distance of $\frac{1}{\sqrt{2}}$ units from point $P(0, 0, 0)$.

Solve

Back to the beginning: the rocket and the station are a case of point-to-line distance; the two beams, of minimum distance between two skew lines: $\frac{16}{\sqrt{83}}\approx1.76$ units in our example, so a connector between them is indeed advisable.

Summary

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