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BAC2 tech · Mathematics
Vectors in space · Metric geometry

Symmetries in space

A laser beam bounces off a flat mirror, an object is reflected in a video game, a mechanical part has to be symmetric about an axis: in all three cases you are asking about the same kind of point, the image (the symmetric point). And in all three cases you calculate it with the same tool as always, the midpoint, changing only what you reflect in: a point, a plane or a line.

Before you start, make sure you know: Midpoint of a line segment (1st Bachillerato)
Before you start, make sure you know: Scalar product (1st Bachillerato)

Symmetry about a point

Points P, M and P' in a straight line, showing that M is the midpoint of segment PP'. P M P' d d M is the midpoint of PP' P' = 2M − P
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In video game design software, an object located at point $P(4, -2, 6)$ must be reflected with respect to a control point (centre of symmetry) located at $M(1, 3, 2)$.…

In video game design software, an object located at point $P(4, -2, 6)$ must be reflected with respect to a control point (centre of symmetry) located at $M(1, 3, 2)$. Calculate the coordinates of the symmetric point $P'$ where the reflected object will appear.

P (4, −2, 6) M (1, 3, 2) P' (x, y, z)
Solve
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It is known that the point $A'(0, 8, -4)$ is the symmetric point of $A(2, -2, 6)$ with respect to a centre of symmetry $C$. Determine the coordinates of this centre $C$…

It is known that the point $A'(0, 8, -4)$ is the symmetric point of $A(2, -2, 6)$ with respect to a centre of symmetry $C$. Determine the coordinates of this centre $C$ using the midpoint relationship.

Solve

Symmetry about a plane

Point P and its symmetric point P' about a plane, with the orthogonal projection M on the plane as the midpoint of segment PP'. P M P'
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Calculate the symmetric point of $Q(3, -5, 7)$ with respect to the coordinate plane $XY$. Hint: Remember that the equation of the plane $XY$ is $z = 0$.

Calculate the symmetric point of $Q(3, -5, 7)$ with respect to the coordinate plane $XY$.

Hint: Remember that the equation of the plane $XY$ is $z = 0$.

Solve
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Given the point $P(1, 0, 1)$, find its symmetric point with respect to the plane $\pi: x + y + z = 5$ by following these steps: 1. Find the line $r$ perpendicular to…

Given the point $P(1, 0, 1)$, find its symmetric point with respect to the plane $\pi: x + y + z = 5$ by following these steps:

  1. Find the line $r$ perpendicular to $\pi$ passing through $P$.

  2. Calculate the point of intersection $M$ between the line $r$ and the plane $\pi$ (foot of the perpendicular).

  3. Use $M$ as the midpoint to find the symmetric point $P'$.

Solve
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Calculate the coordinates of the point $Q$, which is the symmetric point of the point $P(1, 0, 5)$ with respect to the plane with equation $\pi: 2x + y - z - 3 = 0$.…

Calculate the coordinates of the point $Q$, which is the symmetric point of the point $P(1, 0, 5)$ with respect to the plane with equation $\pi: 2x + y - z - 3 = 0$. Briefly explain the steps followed (obtaining the perpendicular line, intersection point and midpoint).

π P(1, 0, 5) Q(x, y, z) M π: 2x + y - z - 3 = 0
Solve

Symmetry about a line

Point P and its symmetric point P' about a line r, with the orthogonal projection M as the midpoint. r M P P'
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Find the point symmetric to $P(1, 2, 3)$ with respect to the line $r$ that coincides with the $OX$-axis.

Find the point symmetric to $P(1, 2, 3)$ with respect to the line $r$ that coincides with the $OX$-axis.

Solve
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Calculate the symmetric point of $B(5, 5, 5)$ with respect to the line $s$ given by: $$s: \begin{cases} x = 1 \\ y = 2 \\ z = \lambda \end{cases}$$ Note: This is a…

Calculate the symmetric point of $B(5, 5, 5)$ with respect to the line $s$ given by:

$$s: \begin{cases} x = 1 \\ y = 2 \\ z = \lambda \end{cases}$$

Note: This is a vertical line, parallel to the $OZ$ axis, that passes through the point $(1, 2, 0)$.

The figure shows the view from above (the $XY$ plane): the line $s$, being vertical, looks like a point, and the projection of $B$ is also shown.

View from above: the vertical line s looks like the point (1, 2) and the point B projects onto (5, 5)123456123456xys: point (1, 2)B (5, 5, 5)
Solve
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Given the line $r$ defined by the parametric equations: $r: \begin{cases} x = 1 + 2\lambda \\ y = \lambda \\ z = -1 + \lambda \end{cases}$ Find the point symmetric to…

Given the line $r$ defined by the parametric equations:

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

Find the point symmetric to $A(3, 2, 1)$ with respect to that line.

r A(3, 2, 1) A' M d d
Solve

Symmetries of whole lines and planes

Reflection of a ray in a plane, showing the incident ray, the point of incidence, the normal to the plane and the reflected ray. incident ray reflected ray
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A laser beam is projected following the path of the line $r: \frac{x-1}{2} = \frac{y+1}{-1} = \frac{z}{1}$. The ray strikes a polished metal surface located in the plane…

A laser beam is projected following the path of the line $r: \frac{x-1}{2} = \frac{y+1}{-1} = \frac{z}{1}$. The ray strikes a polished metal surface located in the plane $XY$ (plane $z=0$). Calculate the equation of the line $s$ containing the reflected ray.

Note: The reflected line is the reflection of the incident line with respect to the plane of incidence.

Solve
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Consider the points $A(1, 1, 1)$ and $B(3, 5, 5)$. Find the equation of the plane $\pi$ acting as a "plane of symmetry" between both points (that is, the plane with…

Consider the points $A(1, 1, 1)$ and $B(3, 5, 5)$. Find the equation of the plane $\pi$ acting as a "plane of symmetry" between both points (that is, the plane with respect to which $A$ and $B$ are symmetric).

X Y Z A(1, 1, 1) B(3, 5, 5) M π : ? Perpendicular bisector plane of AB π ⟂ AB, d(A,π) = d(B,π)
Solve

Summary

See all the exercises in this section