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Secondary 5 · Mathematics
Vectors and geometry · Lines in the plane

Relative Position of Two Lines

Two straight bus routes in Santo Domingo, two beams of light, or two paths on a screen can be related in only three ways: they never cross, they cross at one point, or they are the same line. Knowing which of the three it is, without drawing anything, is what you will learn here.

Three Possible Positions

The three relative positions of two lines in the plane: parallel with no common points, intersecting with one point of intersection, and coincident lines lying on top of each other. parallel intersecting coincident
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Determine the relative position of the lines $r: 2x - 3y + 5 = 0$ and $s: -4x + 6y - 10 = 0$ by directly comparing their coefficients.

Determine the relative position of the lines $r: 2x - 3y + 5 = 0$ and $s: -4x + 6y - 10 = 0$ by directly comparing their coefficients.

Solve

The Criterion with Slopes and Coefficients

Two lines with the same inclination never cross, unless they are the same line. That is why comparing their slopes is enough.

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Given the lines $r: y = \frac{3}{2}x - 1$ and $s: y = \frac{3}{2}x + 4$, state their relative position, justifying your answer using the values of their gradients.

Given the lines $r: y = \frac{3}{2}x - 1$ and $s: y = \frac{3}{2}x + 4$, state their relative position, justifying your answer using the values of their gradients.

Solve

Lines Given in Other Forms

You will not always be given the general equation. The important thing is that from each form you can get a direction vector $\vec{u}=(a,b)$ and a point, or convert it to $Ax+By+C=0$.

If the directions are the same, to decide between parallel and coincident you test whether a point of one line satisfies the equation of the other. If it does, they are coincident; if not, parallel.

Two parallel lines with the same direction vector and a point on one of them that is not on the other. A r s
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Study the relative position of the line $r$, which passes through the points $A(1, 2)$ and $B(3, 5)$, and the line $s: \frac{x-2}{2} = \frac{y+1}{3}$.

Study the relative position of the line $r$, which passes through the points $A(1, 2)$ and $B(3, 5)$, and the line $s: \frac{x-2}{2} = \frac{y+1}{3}$.

Solve
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Determine the relative position of the lines given in parametric form $r: \begin{cases} x = 1 + 2t \\ y = -3 + t \end{cases}$ and…

Determine the relative position of the lines given in parametric form $r: \begin{cases} x = 1 + 2t \\ y = -3 + t \end{cases}$ and $s: \begin{cases} x = 5 - 4\lambda \\ y = 2 - 2\lambda \end{cases}$.

Solve
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An architect designs two straight walkways in a park. Walkway A follows the direction of vector $\vec{u} = (4, -2)$ and passes through the fountain located at $F(0, 5)$.…

An architect designs two straight walkways in a park. Walkway A follows the direction of vector $\vec{u} = (4, -2)$ and passes through the fountain located at $F(0, 5)$. Walkway B is given by the equation $x + 2y - 10 = 0$. Prove analytically that both walkways overlap, being in fact the same line.

Solve

Finding the Point of Intersection

If the lines are intersecting, the point of intersection is the only point that satisfies both equations at the same time. It is found by solving the system of two equations in two unknowns, using substitution, equating, or elimination.

Two intersecting lines in the Cartesian plane with their point of intersection marked and labeled to one side. intersection
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Calculate the exact coordinates of the point of intersection of the intersecting lines $r: x + 2y - 4 = 0$ and $s: 3x - y - 5 = 0$.

Calculate the exact coordinates of the point of intersection of the intersecting lines $r: x + 2y - 4 = 0$ and $s: 3x - y - 5 = 0$.

Solve
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Two exploration drones follow straight-line paths described by the equations $r: x - y + 2 = 0$ and $s: 2x + y - 8 = 0$. Calculate the coordinates of the point in the…

Two exploration drones follow straight-line paths described by the equations $r: x - y + 2 = 0$ and $s: 2x + y - 8 = 0$. Calculate the coordinates of the point in the plane where their paths cross.

Solve
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A ray of light in an optics experiment follows the path described by the line $r: x - 2y + 4 = 0$. A flat mirror is placed exactly on the x-axis ($y = 0$). Determine the…

A ray of light in an optics experiment follows the path described by the line $r: x - 2y + 4 = 0$. A flat mirror is placed exactly on the x-axis ($y = 0$). Determine the coordinates of the point of incidence of the ray on the mirror.

Solve

Intersection When the Lines Come in Another Form

If the lines are not in general form, you first convert them all to $Ax+By+C=0$ and then solve the system.

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In a security system, two laser beams are modelled in a coordinate plane. The first beam follows the line $r: x + y - 6 = 0$ and the second passes through the emitters…

In a security system, two laser beams are modelled in a coordinate plane. The first beam follows the line $r: x + y - 6 = 0$ and the second passes through the emitters located at $P(2, 1)$ and $Q(-2, -1)$. Determine whether the laser beams are parallel, coincident, or intersecting. If they intersect, find the exact point of intersection.

X Y r s P Q
Solve
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The trajectories of two aeroplanes on a two-dimensional radar screen are given by $r: \begin{cases} x = 2 + 3t \\ y = 1 - t \end{cases}$ and…

The trajectories of two aeroplanes on a two-dimensional radar screen are given by $r: \begin{cases} x = 2 + 3t \\ y = 1 - t \end{cases}$ and $s: \begin{cases} x = -1 + 2\lambda \\ y = 4 + \lambda \end{cases}$. Check whether the routes cross and, if so, calculate the point of intersection of the trajectories in the plane.

Solve
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Given the lines $r: \frac{x-1}{2} = \frac{y+2}{3}$ and $s: 3x - 2y + k = 0$, find out whether there is any value of the parameter $k$ for which the lines are coincident.…

Given the lines $r: \frac{x-1}{2} = \frac{y+2}{3}$ and $s: 3x - 2y + k = 0$, find out whether there is any value of the parameter $k$ for which the lines are coincident. Reason your answer analytically.

Solve

Position Depending on a Parameter

If an equation has a letter ($k$, $m$…), the value of that letter decides the position. You impose the parallelism condition with the cross product, solve, and then check the third ratio.

Decision diagram depending on the parameter: if the condition A1B2 equals A2B1 holds the lines are parallel or coincident, and if not, intersecting. If A₁B₂ = A₂B₁ → same direction (parallel or coincident: check C) If A₁B₂ ≠ A₂B₁ → intersecting

Move the slider $m$: when $m=2$ the line $y=mx+1$ is parallel to $y=2x-1$. For any other value they cross.

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Find the value of the parameter $k$ so that the lines $r: kx + 4y - 2 = 0$ and $s: 3x - 2y + 5 = 0$ are strictly parallel.

Find the value of the parameter $k$ so that the lines $r: kx + 4y - 2 = 0$ and $s: 3x - 2y + 5 = 0$ are strictly parallel.

Solve
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Find the value of $m$ so that the lines $r: mx - 6y + 1 = 0$ and $s: 2x + 3y - 4 = 0$ intersect and are not parallel.

Find the value of $m$ so that the lines $r: mx - 6y + 1 = 0$ and $s: 2x + 3y - 4 = 0$ intersect and are not parallel.

Solve
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Discuss the relative position of the lines $r: (m-1)x + 2y - 3 = 0$ and $s: 4x + (m+1)y - 6 = 0$ according to the possible values of the parameter $m$.

Discuss the relative position of the lines $r: (m-1)x + 2y - 3 = 0$ and $s: 4x + (m+1)y - 6 = 0$ according to the possible values of the parameter $m$.

Solve

Discussion with Two Parameters

With two letters you work the same way, but you organize the analysis into cases. First you impose the same direction ($A_1B_2=A_2B_1$); then, for each value found, you check when $C$ also matches.

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Discuss the relative position of the lines $r: ax + y - b = 0$ and $s: 2x - ay + 4 = 0$ by conducting a thorough analysis depending on the simultaneous parameters $a$…

Discuss the relative position of the lines $r: ax + y - b = 0$ and $s: 2x - ay + 4 = 0$ by conducting a thorough analysis depending on the simultaneous parameters $a$ and $b$.

Solve
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The projected orbits of two comets in a spatial plane follow the paths $r: (m+1)x + my - 2 = 0$ and $s: mx + (m-1)y - 1 = 0$. Determine whether there is any moment…

The projected orbits of two comets in a spatial plane follow the paths $r: (m+1)x + my - 2 = 0$ and $s: mx + (m-1)y - 1 = 0$. Determine whether there is any moment (value of $m$) at which the paths are strictly parallel. For which values of $m$ do the orbits cross at a single point?

Solve

Three Lines: Triangles and Concurrency

Three lines in the plane can intersect in several ways:

  • They intersect in pairs at three distinct points: they form a triangle.

  • They pass through the same point (they are concurrent): they do not form a triangle.

  • Two of them are parallel: there is no triangle either.

To find the vertices of a triangle, you solve each pair of lines. To see whether they are concurrent, you find the intersection of two and check that the third passes through that point.

Three lines that intersect in pairs forming a triangle, with its three vertices marked at the intersections.
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Calculate the value of $k$ so that the lines $r: 2x - y + 1 = 0$, $s: x + y - 4 = 0$ and $t: 3x + ky - 2 = 0$ intersect at exactly the same point in the plane.

Calculate the value of $k$ so that the lines $r: 2x - y + 1 = 0$, $s: x + y - 4 = 0$ and $t: 3x + ky - 2 = 0$ intersect at exactly the same point in the plane.

Solve
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Find the coordinates of the vertices of the triangle formed by the intersection of the three lines $r: x - y - 2 = 0$, $s: 2x + 3y - 9 = 0$ and $t: x + 4y - 12 = 0$.

Find the coordinates of the vertices of the triangle formed by the intersection of the three lines $r: x - y - 2 = 0$, $s: 2x + 3y - 9 = 0$ and $t: x + 4y - 12 = 0$.

X Y r s t
Solve
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Three straight roads are modelled on a topographic map using the equations $r: x + 2y - 4 = 0$, $s: 2x - y - 3 = 0$ and $t: ax + y - 2 = 0$. Determine the values of the…

Three straight roads are modelled on a topographic map using the equations $r: x + 2y - 4 = 0$, $s: 2x - y - 3 = 0$ and $t: ax + y - 2 = 0$. Determine the values of the parameter $a$ for which the three roads intersect at a single point or are parallel to each other, preventing a triangle bounded by them from forming.

Solve

Geometry Problems with Lines

Here the ideas of the whole section come together: parallelism, intersections, and conditions on the point of intersection.

A pencil of lines is a family like $(A_1x+B_1y+C_1)+k(A_2x+B_2y+C_2)=0$. All its lines pass through the point where both expressions are zero at the same time.

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A parallelogram has two of its sides on the lines $r: x - 2y + 4 = 0$ and $s: x - 2y - 6 = 0$. Another of its sides lies on the line $t: 2x + y - 2 = 0$. Knowing that…

A parallelogram has two of its sides on the lines $r: x - 2y + 4 = 0$ and $s: x - 2y - 6 = 0$. Another of its sides lies on the line $t: 2x + y - 2 = 0$. Knowing that one of the vertices of the parallelogram is the point $P(4, 4)$, calculate the equation of the line containing the fourth side and determine the exact coordinates of the four vertices.

X Y r s t P(4,4)
Solve
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Determine the value or values of $k$ so that the point of intersection of the intersecting lines $r: 2x + ky - 6 = 0$ and $s: x - y + 2 = 0$ lies on the angle bisector…

Determine the value or values of $k$ so that the point of intersection of the intersecting lines $r: 2x + ky - 6 = 0$ and $s: x - y + 2 = 0$ lies on the angle bisector of the first and third quadrants ($y = x$).

Solve
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Consider the family or pencil of lines defined by the mathematical expression $(2x - y + 1) + k(x + 2y - 3) = 0$. Prove that all the lines that make up this pencil…

Consider the family or pencil of lines defined by the mathematical expression $(2x - y + 1) + k(x + 2y - 3) = 0$. Prove that all the lines that make up this pencil intersect at the same fixed point and calculate its coordinates. Next, find the general equation of the line in this pencil that is parallel to $s: 3x - 4y + 5 = 0$.

Solve

Bounces: the reflection of a point across a line

When a billiard ball bounces off a cushion, the angle of incidence equals the angle of reflection. That is why the path after the bounce passes through the point of impact and through the reflection of a point on the initial path across the cushion. Here is how to calculate that reflection:

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A billiard ball follows the path given by the line $r: x - y + 2 = 0$ and bounces off the cushion represented by the line $s: x + 2y - 8 = 0$. Knowing that the angle of…

A billiard ball follows the path given by the line $r: x - y + 2 = 0$ and bounces off the cushion represented by the line $s: x + 2y - 8 = 0$. Knowing that the angle of incidence is equal to the angle of reflection, calculate the coordinates of the exact point where the ball hits the cushion and find the equation of the line describing the path after the bounce.

Solve
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Given the lines $r: \begin{cases} x = 1 + \lambda \\ y = 2 - 2\lambda \end{cases}$ and $s: ax + by - 5 = 0$, calculate the values of the parameters $a$ and $b$ so that…

Given the lines $r: \begin{cases} x = 1 + \lambda \\ y = 2 - 2\lambda \end{cases}$ and $s: ax + by - 5 = 0$, calculate the values of the parameters $a$ and $b$ so that both lines are identical. Subsequently, find the general equation of a line perpendicular to both that passes through the origin of coordinates.

Solve

Back to the beginning: two straight routes can be parallel, cross at one point, or be the same line, and with slopes, coefficients, or direction vectors you can decide it without drawing.

Summary

See all the exercises in this section