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Vectors in space · Vectors

Linear Dependence and Independence. Bases of R³

Before you start, make sure you know: Linear combinations of vectors. Bases (High School Year 2 (semesters 3 and 4))
Before you start, make sure you know: Systems of three equations in three unknowns (High School Year 2 (semesters 3 and 4))

In the plane, two non-parallel vectors are enough to describe any other vector. In space you need one more: with only two, no matter how different they are, you never leave the plane they form. Think of a drone that moves with three motors: when do three thrust directions let it reach any point in space? This section answers two questions: when are three vectors truly independent? And if they are, how do we use them to give coordinates to any other vector?

Dependence and Independence of Two Vectors

Comparison between two proportional parallel vectors and two vectors in different directions, which are independent u v = 2.5·u u v (independent)
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Two surveillance drones move through space along straight trajectories defined by the vectors $\vec{u} = (2, -4, 6)$ and $\vec{v} = (-1, 2, -3)$. Determine whether the…

Two surveillance drones move through space along straight trajectories defined by the vectors $\vec{u} = (2, -4, 6)$ and $\vec{v} = (-1, 2, -3)$. Determine whether the drones move in parallel directions by analysing the linear dependence of their direction vectors.

Solve
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A laser sensor system requires two beams of light to be parallel. The first beam follows the direction of vector $\vec{d_1} = (3, k, -1)$ and the second follows vector…

A laser sensor system requires two beams of light to be parallel. The first beam follows the direction of vector $\vec{d_1} = (3, k, -1)$ and the second follows vector $\vec{d_2} = (6, -4, -2)$. Find the value of parameter $k$ so that the vectors are linearly dependent and, therefore, the beams are parallel.

Solve
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Consider the points in space $A(1, 2, -1)$, $B(2, 3, 1)$ and $C(3, 4, 3)$. a) Find the vectors $\vec{AB}$ and $\vec{AC}$. b) From their components, decide whether points…

Consider the points in space $A(1, 2, -1)$, $B(2, 3, 1)$ and $C(3, 4, 3)$.

  1. a) Find the vectors $\vec{AB}$ and $\vec{AC}$.
  2. b) From their components, decide whether points $A, B$ and $C$ are collinear.
Solve

The Determinant: Dependence of Three Vectors

For three vectors, comparing components is no longer enough. We need a number that detects whether they "flatten" space into a plane: the determinant. If you remember the determinant method for 2×2 systems, this is the same idea with one more row.

Three non-coplanar vectors forming a parallelepiped with volume, so they are independent u v w det ≠ 0: has volume
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Given the vectors $\vec{u} = (1, 2, 3)$, $\vec{v} = (2, 0, 1)$ and $\vec{w} = (3, 2, 4)$, prove without performing complex calculations (by observing the relationship…

Given the vectors $\vec{u} = (1, 2, 3)$, $\vec{v} = (2, 0, 1)$ and $\vec{w} = (3, 2, 4)$, prove without performing complex calculations (by observing the relationship between their components) that the set $\{\vec{u}, \vec{v}, \vec{w}\}$ is linearly dependent.

Solve
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Check whether the vectors $\vec{e_1} = (1, 0, 0)$, $\vec{e_2} = (1, 1, 0)$ and $\vec{e_3} = (1, 1, 1)$ can form a basis of $\mathbb{R}^3$. To do this, justify whether…

Check whether the vectors $\vec{e_1} = (1, 0, 0)$, $\vec{e_2} = (1, 1, 0)$ and $\vec{e_3} = (1, 1, 1)$ can form a basis of $\mathbb{R}^3$. To do this, justify whether they are linearly independent using the determinant of the matrix they form.

Solve
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State with reasons whether the following statement is true or false: "In three-dimensional space $\mathbb{R}^3$, any set of four non-zero vectors is always linearly…

State with reasons whether the following statement is true or false: "In three-dimensional space $\mathbb{R}^3$, any set of four non-zero vectors is always linearly dependent." Give a simple example to support your answer.

Solve
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Study the linear dependence or independence of the set of vectors $S = \{\vec{a}, \vec{b}, \vec{c}\}$ according to the values of the parameter $m \in \mathbb{R}$, where…

Study the linear dependence or independence of the set of vectors $S = \{\vec{a}, \vec{b}, \vec{c}\}$ according to the values of the parameter $m \in \mathbb{R}$, where $\vec{a} = (m, 1, 1)$, $\vec{b} = (1, m, 1)$, and $\vec{c} = (1, 1, m)$.

Solve

Basis of Space

Perpendicular vectors of the standard basis i, j and k in three-dimensional space i j k
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In a video game graphics engine, the position of an object is defined by a linear combination of two basic vectors: $\vec{a} = (1, 0, 2)$ and $\vec{b} = (0, -1, 3)$.…

In a video game graphics engine, the position of an object is defined by a linear combination of two basic vectors: $\vec{a} = (1, 0, 2)$ and $\vec{b} = (0, -1, 3)$. Calculate the coordinates of the resulting vector $\vec{w}$ if the following operation is applied:

$$\vec{w} = 3\vec{a} - 2\vec{b}$$
Solve
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If a vector $\vec{v}$ has coordinates $(3, -2, 5)$ with respect to the standard basis $\{\vec{i}, \vec{j}, \vec{k}\}$, express it as a linear combination of these unit…

If a vector $\vec{v}$ has coordinates $(3, -2, 5)$ with respect to the standard basis $\{\vec{i}, \vec{j}, \vec{k}\}$, express it as a linear combination of these unit vectors.

X Y Z i j k ? i ? j ? k v(3, −2, 5) v = ? i + ? j + ? k
Solve

Coordinates with Respect to Another Basis

Decomposition of a vector as a linear combination of the vectors of a basis u v w x = a·u + b·v + c·w
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Given the vectors $\vec{u} = (1, 2, -1)$ and $\vec{v} = (0, 1, 2)$, determine the value of the real parameter $k$ such that vector $\vec{w} = (2, k, 0)$ is a linear…

Given the vectors $\vec{u} = (1, 2, -1)$ and $\vec{v} = (0, 1, 2)$, determine the value of the real parameter $k$ such that vector $\vec{w} = (2, k, 0)$ is a linear combination of $\vec{u}$ and $\vec{v}$. What geometrical relationship do the three vectors have for that value of $k$?

X Y Z π u(1,2,-1) v(0,1,2) w(2,?,0) w = α·u + β·v k = ?
Solve
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Consider the points in space $A(1, 0, 0)$, $B(0, 2, 1)$, and $C(2, 0, m)$. a) Find the vector $\vec{AB}$ and the vector $\vec{AC}$. b) Determine the value of $m$ such…

Consider the points in space $A(1, 0, 0)$, $B(0, 2, 1)$, and $C(2, 0, m)$.

  1. a) Find the vector $\vec{AB}$ and the vector $\vec{AC}$.
  2. b) Determine the value of $m$ such that the vector $\vec{v} = (3, -3, -3)$ is linearly dependent on $\vec{AB}$ and $\vec{AC}$.
  3. c) For the value of $m$ found, write $\vec{v}$ as a linear combination of $\vec{AB}$ and $\vec{AC}$.
Geometric representation of coplanarity and linear combination of vector v with respect to AB and ACPlane πCoplanar vectorsLinear dependence in space: coplanar vectorsα · ABβ · ACA(1, 0, 0)B(0, 2, 1)C(2, 0, m)ABACv = (3, −3, −3)Condition: v = α · AB + β · AC
Solve
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Given the vectors of the standard basis $\{\vec{i}, \vec{j}, \vec{k}\}$, the following vectors are defined: $$\vec{u} = \vec{i} + \vec{j} \quad , \quad \vec{v} = \vec{j}…

Given the vectors of the standard basis $\{\vec{i}, \vec{j}, \vec{k}\}$, the following vectors are defined:

$$\vec{u} = \vec{i} + \vec{j} \quad , \quad \vec{v} = \vec{j} + \vec{k} \quad , \quad \vec{w} = \vec{i} + \vec{k}$$

Justify whether the set $\{\vec{u}, \vec{v}, \vec{w}\}$ is a basis of $\mathbb{R}^3$. If a vector $\vec{x}$ has coordinates $(1, 2, 3)$ in the standard basis, what will its coordinates be in the basis $B' = \{\vec{u}, \vec{v}, \vec{w}\}$?

Solve
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Given the vectors in space $\vec{a} = (1, 2, -1)$, $\vec{b} = (0, 1, 2)$, and $\vec{c} = (k, 1, -3)$, answer the following: a) Find the value of $k$ so that the vector…

Given the vectors in space $\vec{a} = (1, 2, -1)$, $\vec{b} = (0, 1, 2)$, and $\vec{c} = (k, 1, -3)$, answer the following:

  1. a) Find the value of $k$ so that the vector $\vec{v} = (3, 4, -7)$ can be expressed as a linear combination of $\vec{a}$, $\vec{b}$, and $\vec{c}$ such that the three vectors are linearly dependent.
  1. b) If $k = 2$, do they form a basis of $\mathbb{R}^3$? If so, calculate the coordinates of the vector $\vec{w} = (1, 1, 1)$ with respect to this basis $B = \{ \vec{a}, \vec{b}, \vec{c} \}$.
Solve

Applications: Coplanarity and Proofs

Two frequent uses of linear dependence:

  • Collinear points: $A,B,C$ are collinear if $\overrightarrow{AB}$ and $\overrightarrow{AC}$ are dependent (proportional).

  • Coplanar points: $A,B,C,D$ lie in the same plane if $\overrightarrow{AB}$, $\overrightarrow{AC}$ and $\overrightarrow{AD}$ are dependent, that is, if their determinant equals $0$.

To prove independence without numbers, we assume $\alpha\vec u+\beta\vec v+\gamma\vec w=\vec 0$, use the fact that $\{\vec u,\vec v,\vec w\}$ is independent (so each resulting coefficient must be zero), and check that $\alpha=\beta=\gamma=0$.

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A surveillance drone moves along straight trajectories defined by vectors. If the direction vectors of three drones are $\vec{d_1} = (1, 3, a)$, $\vec{d_2} = (4, 1, 2)$…

A surveillance drone moves along straight trajectories defined by vectors. If the direction vectors of three drones are $\vec{d_1} = (1, 3, a)$, $\vec{d_2} = (4, 1, 2)$ and $\vec{d_3} = (a, -1, 0)$, calculate the values of $a$ for which the trajectories of the three drones would be contained in planes parallel to each other (that is, the vectors are coplanar).

Solve
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Determine whether the points $P(1, 2, -1)$, $Q(3, 1, 1)$, $R(0, 5, -2)$ and $S(k, 0, 4)$ are coplanar depending on the value of $k$. To carry out this analysis, use the…

Determine whether the points $P(1, 2, -1)$, $Q(3, 1, 1)$, $R(0, 5, -2)$ and $S(k, 0, 4)$ are coplanar depending on the value of $k$. To carry out this analysis, use the linear dependence of the vectors originating at $P$ with endpoints at the other points.

π k P(1, 2, −1) Q(3, 1, 1) R(0, 5, −2) S(k, 0, 4)
Solve
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Consider the points $A(1, 0, 1)$, $B(0, 2, 1)$ and $C(2, 1, 0)$. a) Find a point $D$ such that vector $\vec{AD}$ is linearly dependent on vector $\vec{BC}$ and has a…

Consider the points $A(1, 0, 1)$, $B(0, 2, 1)$ and $C(2, 1, 0)$.

  1. a) Find a point $D$ such that vector $\vec{AD}$ is linearly dependent on vector $\vec{BC}$ and has a magnitude of 6. Is the solution unique?
  1. b) Determine whether point $E(5, -1, 4)$ belongs to the plane defined by vectors $\vec{AB}$ and $\vec{AC}$. Justify your answer using the concepts of linear dependence and basis.
Solve

Back to the beginning: the drone's three motors let it maneuver throughout space when their directions are independent, that is, when $\det(\vec u,\vec v,\vec w)\neq 0$. Then they form a basis and any vector can be expressed uniquely as $a\vec u+b\vec v+c\vec w$, by solving a system to find $(a,b,c)$.

Summary

See all the exercises in this section