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Parallel Lines and Proportional Segments

Before you start, make sure you know: Ratio (Math 6)
Before you start, make sure you know: Linear equations (Math 6)

How tall is the flagpole in front of your school? You can't climb it, and your tape measure is only 25 feet long. But you do have a sunny day, a friend, and a way to measure shadows. Parallel lines (like the sun's rays) can turn that into a simple proportion, and that is exactly what this page is about.

Parallel lines and proportional segments

Picture a ladder lying across two roads that meet at a point. The rungs are parallel. The closer the roads are to each other, the shorter each rung is, but the rungs keep the same proportions. That is the big idea.

Two transversals cut by three parallel lines. The segments AB and BC on one transversal are proportional to A'B' and B'C' on the other. ABC A′B′C′ 4 in. 6 in. 2 in. x parallel lines

The proportion works in any units, as long as the two segments on the same line use the same unit.

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Calculate the value of the unknown $x$ in the following figure, given that the horizontal segments are parallel to each other.

Calculate the value of the unknown $x$ in the following figure, given that the horizontal segments are parallel to each other.

5 in 8 in 3 in x
Solve
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In the following diagram, lines $r$ and $s$ are intersected by three parallel lines ($a$, $b$ and $c$). Use the data provided to calculate the length of the unknown…

In the following diagram, lines $r$ and $s$ are intersected by three parallel lines ($a$, $b$ and $c$). Use the data provided to calculate the length of the unknown segment $x$.

a b c r s 4 in 6 in x 7.5 in
Solve
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In a design structure for a pedestrian bridge, three horizontal reinforcing beams have been placed parallel to each other. The beams are joined by two oblique steel tie…

In a design structure for a pedestrian bridge, three horizontal reinforcing beams have been placed parallel to each other. The beams are joined by two oblique steel tie rods, as shown in the figure. Knowing that the segments determined on the first tie rod measure $x$ and $x + 2$ feet, and the corresponding segments on the second tie rod measure $3$ and $4$ feet, calculate the value of $x$ and the total length of the first tie rod.

x x + 2 3 ft 4 ft
Solve

Nested triangles with a parallel side

Draw a line $DE$ parallel to the side $BC$ of triangle $ABC$. The small triangle $ADE$ sits inside the big one: it shares the vertex $A$ and its opposite side is parallel to $BC$. The two triangles are similar: same shape, different size, and all corresponding sides are in the same ratio, the scale factor.

Triangle ABC with a segment DE parallel to BC, forming a smaller similar triangle ADE that shares vertex A. A DE BC DE ∥ BC
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Two triangles are in Thales's position. The small triangle has sides measuring $3\text{ cm}$, $4\text{ cm}$ and $5\text{ cm}$. If the side corresponding to $3\text{ cm}$…

Two triangles are in Thales's position. The small triangle has sides measuring $3\text{ cm}$, $4\text{ cm}$ and $5\text{ cm}$. If the side corresponding to $3\text{ cm}$ in the large triangle measures $9\text{ cm}$, what will the other two sides of the larger triangle measure?

Solve
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A young graphic designer is creating a logo that includes two triangles in Thales's position. Knowing that segment $DE$ is parallel to $BC$, calculate the lengths of…

A young graphic designer is creating a logo that includes two triangles in Thales's position. Knowing that segment $DE$ is parallel to $BC$, calculate the lengths of segments $AD$ and $BC$ if we know that $AB = 12$ in., $AE = 5$ in., $AC = 15$ in., and $DE = 3$ in.

A B C D E
Solve
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A carpenter is building a designer shelving unit in the shape of a right triangle. They have placed two horizontal shelves parallel to the base. The base of the shelving…

A carpenter is building a designer shelving unit in the shape of a right triangle. They have placed two horizontal shelves parallel to the base. The base of the shelving unit measures $1,2$ feet. The first shelf is located at a height of $40$ in. from the base and is $90$ in. long. The second shelf is $60$ in. long. At what height from the base is the second shelf located? What is the total height of the shelving unit? (Express all results in inches).

120 in 90 in 60 in 40 in h₂ = ? H = ?
Solve

Are the lines parallel?

The theorem also works backward. If the pieces are in proportion, the lines are parallel. If they are not in proportion, the lines are not parallel.

Triangle ABC with D on AB and E on AC. AD is 6, DB is 9, AE is 8, EC is 12. Is DE parallel to BC? A DE BC 6 9 8 12 DE ∥ BC ?
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A carpenter is building a shelving unit and wants to check if the two shelves are parallel. To do this, he measures the distances on the side supports. On the left side,…

A carpenter is building a shelving unit and wants to check if the two shelves are parallel. To do this, he measures the distances on the side supports. On the left side, the distance between the floor and the first shelf is $30\text{ cm}$, and between the first and second there is $40\text{ cm}$. On the right side, the distance to the floor is $45\text{ cm}$ and between shelves there is $60\text{ cm}$. Are the shelves parallel? Justify your answer using the proportion from Thales's theorem.

Floor Shelf 1 Shelf 2 30 in 40 in 45 in 60 in
Solve
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Check if lines $r$ and $s$ in the following figure are parallel. To do this, verify if the proportionality relationship is met between the segments they determine on the…

Check if lines $r$ and $s$ in the following figure are parallel. To do this, verify if the proportionality relationship is met between the segments they determine on the intersecting transversals. Justify your answer with the necessary calculations.

r s 4,2 8,4 3,5 7
Solve

Measuring heights with shadows

At the same moment of the day, the sun's rays hit every object at the same angle: they are parallel. So an object and its shadow form a right triangle that is similar to the right triangle of any other object and its shadow.

A flagpole and a student with their shadows. Parallel sun rays make two similar right triangles. h = ? 20 ft 5 ft 4 ft parallel sun rays
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A young man of height $1,5\text{ meters}$ casts a shadow $1\text{ metro}$ long. At that same moment, the shadow of a street lamp measures $4\text{ meters}$. What is the…

A young man of height $1,5\text{ meters}$ casts a shadow $1\text{ metro}$ long. At that same moment, the shadow of a street lamp measures $4\text{ meters}$. What is the height of the street lamp? Draw a diagram with two right triangles to visualize the situation.

α 1.5 ft 1 ft α h 4 ft
Solve
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In a park, a tree of height $2\text{ meters}$ casts a shadow of $3,5\text{ meters}$. A few feet away there is a statue whose shadow measures exactly $7\text{ meters}$.…

In a park, a tree of height $2\text{ meters}$ casts a shadow of $3,5\text{ meters}$. A few feet away there is a statue whose shadow measures exactly $7\text{ meters}$. What is the height of the statue? Use the similarity of triangles established by Thales's theorem.

α 2 ft 3.5 ft α x 7 ft
Solve
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To measure the height of a basketball hoop, an 8th grade student places a vertical stick $1,5$ feet high so that the end of its shadow exactly coincides with the end of…

To measure the height of a basketball hoop, an 8th grade student places a vertical stick $1,5$ feet high so that the end of its shadow exactly coincides with the end of the hoop's shadow. If the stick's shadow measures $2,1$ feet and the distance from the base of the stick to the base of the hoop is $2,2$ feet, what is the height of the hoop?

2.1 ft stick shadow 2.2 ft 1.5 ft h
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Dividing a segment into equal or proportional parts

Parallel lines also let you build things with a ruler and compass. You can split a segment into any number of equal parts without guessing by eye.

Dividing segment AB into three equal parts: an auxiliary ray from A with three equal marks, then lines parallel to the line from the last mark to B. AB auxiliary ray with 3 equal marks

To divide a segment in a ratio like $2:3$, add the numbers and make that many marks: $2 + 3 = 5$ marks. Connect the 5th mark to $B$, and draw the parallel through the 2nd mark. It hits $AB$ at a point $P$ with $AP : PB = 2 : 3$.

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Draw a segment $AB$ measuring $10\text{ cm}$ in your notebook. Using Thales's theorem method, divide it exactly into $4$ equal parts. Briefly describe the steps you have…

Draw a segment $AB$ measuring $10\text{ cm}$ in your notebook. Using Thales's theorem method, divide it exactly into $4$ equal parts. Briefly describe the steps you have followed (drawing the auxiliary ray, equal divisions and joining the endpoints).

Solve
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Divide a segment of $7\text{ cm}$ into two parts such that their ratio is $2$ to $3$. To do this, use an auxiliary ray and mark $5$ equal divisions ($2+3$). What is the…

Divide a segment of $7\text{ cm}$ into two parts such that their ratio is $2$ to $3$. To do this, use an auxiliary ray and mark $5$ equal divisions ($2+3$). What is the length of each of the two new segments obtained on the original one?

Solve
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Graphically divide a segment $AB$ of length $9$ in. into three parts that are proportional to the numbers $2$, $3$ and $4$. Briefly explain the steps taken based on…

Graphically divide a segment $AB$ of length $9$ in. into three parts that are proportional to the numbers $2$, $3$ and $4$. Briefly explain the steps taken based on Thales's Theorem.

Solve

The fourth proportional

Sometimes you have three lengths $a$, $b$, $c$ and you want a fourth length $x$ so that

$$\frac{a}{b} = \frac{c}{x}.$$

That $x$ is the fourth proportional. Numerically it is easy: $x = \dfrac{b \cdot c}{a}$. Geometrically, it comes from two rays with a common starting point.

Construction of the fourth proportional: on one ray mark OA equal to a and OB equal to b, on the other ray mark OC equal to c; the parallel to AC through B meets the second ray at D, and OD equals x. O A B C D a = 6 b = 9 c = 4
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Find the fourth proportional of three segments whose lengths are $a = 8$ in., $b = 12$ in., and $c = 10$ in. Perform both the numerical calculation and the geometric…

Find the fourth proportional of three segments whose lengths are $a = 8$ in., $b = 12$ in., and $c = 10$ in. Perform both the numerical calculation and the geometric construction using Thales's Theorem on two concurrent rays.

Solve

Real-world problems with hidden triangles

Many problems don't show the parallel lines. You have to find the triangles yourself. Use this routine.

A lamp post and a student whose shadow ends at the same point on the ground, forming two similar right triangles that share the angle at the tip of the shadow. H = ? 5 ft 12 ft 8 ft light

Sometimes the two triangles meet tip to tip, like a bow tie. This happens when you look at the bottom of something through a point on the ground.

Cross-section of a well. A person stands 2 feet from the edge with eyes 5 feet above the ground, and the line of sight touches the edge and reaches the bottom of the far wall, forming two similar right triangles. 5 ft 2 ft 3 ft D = ? line of sight
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A stepladder is leaning against a wall. The rungs are parallel to the ground. The first rung is $30$ in. from the ground following the inclination of the ladder, and the…

A stepladder is leaning against a wall. The rungs are parallel to the ground. The first rung is $30$ in. from the ground following the inclination of the ladder, and the second is another $30$ in. from the first. If the horizontal distance from the base of the ladder to the wall is $1,2$ feet, what is the horizontal distance from the base to the vertical of the second rung?

Solve
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Two buildings located on the same street cast shadows that overlap at a given time of day. Building A has a height of $18$ feet and casts a shadow of $12$ feet. Building…

Two buildings located on the same street cast shadows that overlap at a given time of day. Building A has a height of $18$ feet and casts a shadow of $12$ feet. Building B is located behind it so that its shadow ends at exactly the same point as that of building A. If the distance between the base of building A and the end of its shadow is $12$ feet, and the distance between the two buildings is $10$ feet, calculate the height of building B.

10 ft 12 ft 18 ft H = ?
Solve
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To measure the depth of a circular well $2$ feet in diameter, a person $1,70$ feet tall stands $1,2$ feet from the edge of the well. From that position, their eyes…

To measure the depth of a circular well $2$ feet in diameter, a person $1,70$ feet tall stands $1,2$ feet from the edge of the well. From that position, their eyes (located $1,60$ feet from the ground) see the edge of the well aligned with the bottom end of the opposite wall. Draw a diagram of the situation and calculate the depth of the well using Thales's Theorem.

Solve

Parallel segments inside a trapezoid

A trapezoid has two parallel sides (the bases). The legs are like two transversals, so any segment drawn parallel to the bases follows the proportional-segments property. In particular, a segment drawn halfway up has a length exactly halfway between the two bases.

A right trapezoid with bases 8 inches and 5 inches. A segment parallel to the bases halfway up the height has length 6.5 inches. 8 in. 5 in. ? in. halfway up

Some trapezoid problems come from a map. A scale of $1:2{,}500$ means $1$ in. on the map stands for $2{,}500$ in. in real life. Multiply by $2{,}500$ to go from the map to real life, and divide by $2{,}500$ to go from real life to the map. Remember that $12$ in. $= 1$ ft before you change units.

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On a map with a scale of $1:2.500$, a plot of land in the shape of a right trapezoid has a larger base of $8$ in. and a smaller base of $5$ in. If in reality the height…

On a map with a scale of $1:2.500$, a plot of land in the shape of a right trapezoid has a larger base of $8$ in. and a smaller base of $5$ in. If in reality the height of this land is $100$ feet, calculate:

  1. The actual length of the bases in feet.

  2. The measurement of the height on the map in inches.

  3. If we draw a line parallel to the bases that divides the height of the map into exactly two equal parts, how long will that new line measure on the map?

Scale 1:2,500 5 in 8 in h = ? (100 ft actual) x = ?
Solve

Back to the flagpole: Aaliyah's height and shadow ($5$ ft and $4$ ft) and the flagpole's shadow ($20$ ft) were measured at the same moment, so the sun's parallel rays made two similar right triangles. The ratio height : shadow is the same for both, and that gives a flagpole height of $25$ ft, with no climbing.

Summary

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