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Secondary 5 · Mathematics
Vectors and geometry · Vectors

Linear Combinations of Vectors. Bases

When you locate a point at $(3, 5)$ on the coordinate plane, you are really saying something more specific: that to get there you move $3$ times the vector $(1, 0)$ and $5$ times the vector $(0, 1)$. Those two vectors form a basis, but they are not the only possible ones. Can any pair of vectors describe the whole plane? We'll find out in the section on the basis.

Linear combination

Vector w is the diagonal of the parallelogram built with a·u and b·v a·u b·v w = a·u + b·v
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Calculate the coordinates of the vector resulting from the linear combination $3\vec{a} - 2\vec{b}$, knowing that $\vec{a} = (1, 4)$ and $\vec{b} = (-2, 5)$.

Calculate the coordinates of the vector resulting from the linear combination $3\vec{a} - 2\vec{b}$, knowing that $\vec{a} = (1, 4)$ and $\vec{b} = (-2, 5)$.

Solve
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A logistics company uses two types of vans. The load capacity of type A vans is represented by the vector $\vec{a} = (3, 2)$, indicating 3 pallets of fruit and 2 of…

A logistics company uses two types of vans. The load capacity of type A vans is represented by the vector $\vec{a} = (3, 2)$, indicating 3 pallets of fruit and 2 of vegetables. Type B is represented by $\vec{b} = (1, 4)$. If a shopping center places an exact order of 11 pallets of fruit and 14 of vegetables, represented by $\vec{p} = (11, 14)$, find the scalars of the linear combination $\vec{p} = x\vec{a} + y\vec{b}$ to determine how many trips each van needs to make.

Solve
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The attached plane shows the vectors $\vec{u}$ and $\vec{v}$. Determine the coordinates of the resultant vector $\vec{z} = 2\vec{u} - \vec{v}$ in the standard basis.

The attached plane shows the vectors $\vec{u}$ and $\vec{v}$. Determine the coordinates of the resultant vector $\vec{z} = 2\vec{u} - \vec{v}$ in the standard basis.

X Y u v
Solve

Linear dependence and independence

Comparison between two parallel, linearly dependent vectors and two non-parallel, linearly independent vectors dependent (parallel) independent
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Given the vectors $\vec{u} = (2, -3)$ and $\vec{v} = (-4, 6)$, mathematically justify whether they can form a basis for the vector space $\mathbb{R}^2$.

Given the vectors $\vec{u} = (2, -3)$ and $\vec{v} = (-4, 6)$, mathematically justify whether they can form a basis for the vector space $\mathbb{R}^2$.

Solve
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Determine the value of the parameter $m$ such that the vectors $\vec{p} = (m, 4)$ and $\vec{q} = (3, -2)$ are linearly dependent.

Determine the value of the parameter $m$ such that the vectors $\vec{p} = (m, 4)$ and $\vec{q} = (3, -2)$ are linearly dependent.

Solve
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To guarantee the stability of a node in a planar lattice structure, the direction vectors of two concurrent bars must be linearly independent. The design vectors are…

To guarantee the stability of a node in a planar lattice structure, the direction vectors of two concurrent bars must be linearly independent. The design vectors are $\vec{d}_1 = (k+1, 3)$ and $\vec{d}_2 = (2, k)$. Determine for which values of the parameter $k$ the node is unstable (the vectors do not form a basis). Subsequently, assuming $k=0$, express the external load vector $\vec{P} = (4, -6)$ as a linear combination of $\vec{d}_1$ and $\vec{d}_2$.

Solve

Basis of the plane

And now the answer to the opening question: not just any pair of vectors will do, but any non-parallel pair. With two independent vectors you can reach every point of the plane.

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Reason whether three non-zero vectors in the plane, for example $\vec{u} = (1, 1)$, $\vec{v} = (-1, 2)$ and $\vec{w} = (3, 0)$, can be simultaneously linearly…

Reason whether three non-zero vectors in the plane, for example $\vec{u} = (1, 1)$, $\vec{v} = (-1, 2)$ and $\vec{w} = (3, 0)$, can be simultaneously linearly independent.

Solve
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Given the parallelogram $ABCD$ in the figure, express the vectors corresponding to the diagonals $\vec{AC}$ and $\vec{BD}$ as a linear combination of the side vectors…

Given the parallelogram $ABCD$ in the figure, express the vectors corresponding to the diagonals $\vec{AC}$ and $\vec{BD}$ as a linear combination of the side vectors $\vec{u} = \vec{AB}$ and $\vec{v} = \vec{AD}$.

A B C D u v
Solve
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In the regular hexagon $ABCDEF$ with center $O$ shown in the figure, the basis vectors $\vec{u} = \vec{AB}$ and $\vec{v} = \vec{AF}$ are defined. Express the vectors…

In the regular hexagon $ABCDEF$ with center $O$ shown in the figure, the basis vectors $\vec{u} = \vec{AB}$ and $\vec{v} = \vec{AF}$ are defined. Express the vectors $\vec{AC}$, $\vec{AD}$, $\vec{AE}$ and $\vec{BC}$ as a linear combination of the basis $B = \{\vec{u}, \vec{v}\}$.

A B C D E F O u v
Solve

Orthogonal and orthonormal bases

Two perpendicular vectors u and v forming a right angle at the origin u v
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Check whether the set of vectors $B = \{(-1, 2), (2, 1)\}$ forms an orthogonal basis for the plane.

Check whether the set of vectors $B = \{(-1, 2), (2, 1)\}$ forms an orthogonal basis for the plane.

Solve
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Check whether the vectors $\vec{u} = (3, 0)$ and $\vec{v} = (0, -2)$ form an orthogonal basis for $\mathbb{R}^2$.

Check whether the vectors $\vec{u} = (3, 0)$ and $\vec{v} = (0, -2)$ form an orthogonal basis for $\mathbb{R}^2$.

Solve
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Determine a vector $\vec{v}$ such that the set $B = \{\vec{u}, \vec{v}\}$ forms an orthonormal basis, given that $\vec{u} = \left(\frac{3}{5}, \frac{-4}{5}\right)$. Is…

Determine a vector $\vec{v}$ such that the set $B = \{\vec{u}, \vec{v}\}$ forms an orthonormal basis, given that $\vec{u} = \left(\frac{3}{5}, \frac{-4}{5}\right)$. Is the solution unique? Justify your answer.

Solve

Dot product and angle between vectors

To work with orthogonality and magnitudes you need the dot product (or scalar product). Here you only use it; section ESC-PROD studies it carefully.

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Find the values of $a$ and $b$ so that the vectors $\vec{u} = (a, 3)$ and $\vec{v} = (-2, b)$ form an orthogonal basis, given also that the magnitude of the vector…

Find the values of $a$ and $b$ so that the vectors $\vec{u} = (a, 3)$ and $\vec{v} = (-2, b)$ form an orthogonal basis, given also that the magnitude of the vector $\vec{u}$ is exactly $5$.

Solve
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Let $\vec{u}$ and $\vec{v}$ be two non-zero vectors such that the angle between them is $60^\circ$, with $|\vec{u}| = 2$ and $|\vec{v}| = 3$. If the vector…

Let $\vec{u}$ and $\vec{v}$ be two non-zero vectors such that the angle between them is $60^\circ$, with $|\vec{u}| = 2$ and $|\vec{v}| = 3$. If the vector $\vec{w} = m\vec{u} + n\vec{v}$ is defined, calculate the relation that must exist between the parameters $m$ and $n$ so that the vector $\vec{w}$ is perpendicular to the vector $\vec{u}$.

Solve
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The arm of an industrial robot moves in a plane defined by a basis $B = \{\vec{e}1, \vec{e}2\}$ which is orthogonal but not orthonormal, where $|\vec{e}1| = 2$ and…

The arm of an industrial robot moves in a plane defined by a basis $B = \{\vec{e}_1, \vec{e}_2\}$ which is orthogonal but not orthonormal, where $|\vec{e}_1| = 2$ and $|\vec{e}_2| = 4$. A path command instructs it to move according to the vector $\vec{v} = 3\vec{e}_1 - \frac{1}{2}\vec{e}_2$. Calculate the exact magnitude of the displacement vector $\vec{v}$ and the cosine of the angle it forms with the basis vector $\vec{e}_1$.

Solve

Change of basis

Vector w is the sum of 4 times c1 and minus c2, with oblique, non-orthogonal axes c1 c2 4·c1 −c2 w = 4·c1 − c2
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An underwater drone moves in an environment with ocean currents. Its autonomous navigation system uses a basis formed by the vectors $\vec{c}1 = (1, 2)$ and…

An underwater drone moves in an environment with ocean currents. Its autonomous navigation system uses a basis formed by the vectors $\vec{c}_1 = (1, 2)$ and $\vec{c}_2 = (-2, 3)$. The drone records a displacement $\vec{w}$ whose coordinates in this basis are $(4, -1)_B$. Calculate the coordinates of the vector $\vec{w}$ in the standard basis.

Solve
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Two tugboats pull a disabled ship with forces $\vec{F}1 = (300, 400)$ N and $\vec{F}2 = (400, -300)$ N. A third tugboat joins by applying a force…

Two tugboats pull a disabled ship with forces $\vec{F}_1 = (300, 400)$ N and $\vec{F}_2 = (400, -300)$ N. A third tugboat joins by applying a force $\vec{F}_3 = (1000, 500)$ N. Express the force $\vec{F}_3$ as a linear combination of the vectors $\vec{F}_1$ and $\vec{F}_2$, finding the coordinates of $\vec{F}_3$ in the basis $B = \{\vec{F}_1, \vec{F}_2\}$.

Solve
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Prove analytically that if $B = \{\vec{u}, \vec{v}\}$ is a basis of vectors in the plane, then the set $B' = \{\vec{u} + \vec{v}, \vec{u} - \vec{v}\}$ also forms a…

Prove analytically that if $B = \{\vec{u}, \vec{v}\}$ is a basis of vectors in the plane, then the set $B' = \{\vec{u} + \vec{v}, \vec{u} - \vec{v}\}$ also forms a basis. If a vector $\vec{w}$ has coordinates $(3, 5)$ in the basis $B$, find its coordinates in the new basis $B'$.

Solve

Problems with bases and coordinates

When one basis is defined in terms of another, or when the data mix points and coordinates, the approach is the same: substitute and solve a system.

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In the physics engine of a video game, the movement of a character is defined in a basis $B = \{\vec{u}, \vec{v}\}$, where $\vec{u} = (2, 1)$ and $\vec{v} = (-1, 3)$. If…

In the physics engine of a video game, the movement of a character is defined in a basis $B = \{\vec{u}, \vec{v}\}$, where $\vec{u} = (2, 1)$ and $\vec{v} = (-1, 3)$. If the character must move to the point indicated by the vector $\vec{d} = (8, -3)$ relative to the standard basis, what will be the coordinates of the displacement vector in the basis $B$?

Solve
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In the study of a two-dimensional crystal lattice, two bases are defined for the plane: the primitive basis $B_1 = \{\vec{a}1, \vec{a}2\}$ and a secondary basis…

In the study of a two-dimensional crystal lattice, two bases are defined for the plane: the primitive basis $B_1 = \{\vec{a}_1, \vec{a}_2\}$ and a secondary basis $B_2 = \{\vec{b}_1, \vec{b}_2\}$. Knowing that $\vec{b}_1 = 2\vec{a}_1 - \vec{a}_2$ and $\vec{b}_2 = \vec{a}_1 + 3\vec{a}_2$, find the coordinates in the basis $B_1$ of a point defect whose position vector in the basis $B_2$ is $\vec{d} = (-1, 2)_{B_2}$.

Solve
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Find all vectors $\vec{x} = (x_1, x_2)$ that, when expressed as a linear combination of the basis $B = \{(1, -1), (2, 1)\}$, have coordinates $(a, b)B$ such that $a = b$…

Find all vectors $\vec{x} = (x_1, x_2)$ that, when expressed as a linear combination of the basis $B = \{(1, -1), (2, 1)\}$, have coordinates $(a, b)_B$ such that $a = b$ and, furthermore, satisfy that their magnitude in the standard basis is exactly $\sqrt{45}$.

Solve
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Given the points $A(1, 2)$, $B(4, -1)$ and $C(x, y)$, determine the coordinates of the point $C$ knowing that the vectors $\vec{u} = \vec{AB}$ and $\vec{v} = \vec{AC}$…

Given the points $A(1, 2)$, $B(4, -1)$ and $C(x, y)$, determine the coordinates of the point $C$ knowing that the vectors $\vec{u} = \vec{AB}$ and $\vec{v} = \vec{AC}$ form a basis of the plane in which the vector $\vec{w} = (0, -6)$ has coordinates $(2, -1)_B$.

Solve

Summary

See all the exercises in this section