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Applications of the Pythagorean Theorem

Before you start, make sure you know: Square root (Secondary Year 1)
Before you start, make sure you know: Triangles (Secondary Year 1)

You bought a tablet and the salesperson says it is "10 inches." Is that the width, the height, or what? The Pythagorean theorem almost never shows up alone: you have to find the right triangle hidden in a screen, a roof, or a guy wire. Many times you also have to combine it with similarity. At the end of the page we will settle the question about inches.

The diagonal of a rectangle: screens and photos

In any rectangle, the diagonal, the width, and the height form a right triangle. The diagonal is the hypotenuse, so $d^2 = \text{width}^2 + \text{height}^2$.

Rectangle with its diagonal marked as the hypotenuse of a right triangle formed together with the width and the height. width height diagonal
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A tablet has a rectangular screen with dimensions of $24 \text{ cm}$ base and $7 \text{ cm}$ height. Calculate the length of its diagonal applying Pythagoras' theorem.

A tablet has a rectangular screen with dimensions of $24 \text{ cm}$ base and $7 \text{ cm}$ height. Calculate the length of its diagonal applying Pythagoras' theorem.

24 cm 7 cm d
Solve
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Marta has bought a $50$ inch television. Knowing that $1$ inch is equivalent to $2.54 \text{ cm}$, the diagonal of the screen measures exactly $127 \text{ cm}$. If the…

Marta has bought a $50$ inch television. Knowing that $1$ inch is equivalent to $2.54 \text{ cm}$, the diagonal of the screen measures exactly $127 \text{ cm}$. If the width of the television is $110 \text{ cm}$, what is the height of the screen? Use Pythagoras' theorem and round your answer to one decimal place.

50" 110 cm x 127 cm
Solve

Cables, antennas, and zip lines

A guy wire, the antenna, and the stretch of ground between them form a right triangle. The wire is the hypotenuse.

Vertical pole on the ground held by a slanted guy wire that acts as the hypotenuse. 12 m 5 m wire
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To secure a radio mast, a guy wire has been attached from its top to the ground. If the mast is $12 \text{ m}$ high and the wire is anchored to the ground at a distance…

To secure a radio mast, a guy wire has been attached from its top to the ground. If the mast is $12 \text{ m}$ high and the wire is anchored to the ground at a distance of $5 \text{ m}$ from the base, how long is the wire?

12 m 5 m L
Solve
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To cross a small ravine in an adventure park, a zip line $25 \text{ m}$ long has been installed. The starting point is on a tower at a height of $18 \text{ m}$ and the…

To cross a small ravine in an adventure park, a zip line $25 \text{ m}$ long has been installed. The starting point is on a tower at a height of $18 \text{ m}$ and the end point is on another platform at a height of $3 \text{ m}$. What is the horizontal distance separating both structures?

18 m 3 m 15 m (18 - 3) d = ? 25 m
Solve

Roofs and isosceles triangles

A gable roof has the shape of an isosceles triangle. Its height falls exactly on the midpoint of the base. Then the height, half of the base, and one of the equal sides form a right triangle.

Roof shaped like an isosceles triangle divided by its perpendicular height into two right triangles. half of the base height side
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The roof of a garden shed is shaped like an isosceles triangle. The base of the triangle measures $6 \text{ meters}$ and the equal sides measure $5 \text{ meters}$ each.…

The roof of a garden shed is shaped like an isosceles triangle. The base of the triangle measures $6 \text{ meters}$ and the equal sides measure $5 \text{ meters}$ each. Calculate the height of the roof.

h 5 m 5 m 3 m 6 m
Solve

Rhombuses and hexagons

The diagonals of a rhombus intersect perpendicularly and at their midpoints. They divide the rhombus into four equal right triangles. In a regular hexagon, the side, the apothem, and half of the side also form a right triangle.

Rhombus divided by its two perpendicular diagonals into four equal right triangles. diagonal diagonal side
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Calculate the perimeter of a rhombus whose diagonals measure $16 \text{ cm}$ and $12 \text{ cm}$. (Hint: Remember that the diagonals of a rhombus are perpendicular and…

Calculate the perimeter of a rhombus whose diagonals measure $16 \text{ cm}$ and $12 \text{ cm}$. (Hint: Remember that the diagonals of a rhombus are perpendicular and bisect each other, forming four congruent right-angled triangles).

16 cm 12 cm 8 cm 6 cm L
Solve
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Calculate the perimeter and the area of a regular hexagon whose side measures $8 \text{ cm}$. To find the area, remember to first calculate the apothem using Pythagoras'…

Calculate the perimeter and the area of a regular hexagon whose side measures $8 \text{ cm}$. To find the area, remember to first calculate the apothem using Pythagoras' theorem in one of the equilateral triangles that form the hexagon.

4 cm Side = 8 cm ap 8 cm
Solve

The diagonal of a box

Does a long object fit inside a box? The longest thing that fits goes from one corner to the opposite corner: it is the space diagonal. You get it by using Pythagoras twice. First the diagonal of the base, $d_b^2 = a^2 + b^2$. Then that diagonal and the height $c$ form another right triangle whose hypotenuse is the diagonal of the box.

Rectangular prism with the diagonal of the base and the space diagonal, which form two chained right triangles. length height
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A gamer wants to fit their $52\text{ cm}$ long mechanical keyboard inside a carrying case they bought. The case is a rectangular prism with internal dimensions of…

A gamer wants to fit their $52\text{ cm}$ long mechanical keyboard inside a carrying case they bought. The case is a rectangular prism with internal dimensions of $48\text{ cm}$ long, $18\text{ cm}$ wide and $10\text{ cm}$ high. Will the keyboard fit in the box if placed connecting two opposite vertices (the space diagonal)? Calculate the length of the space diagonal inside the box to justify your answer.

D 48 cm 10 cm 18 cm
Solve

Scale, enlargements, and reductions

Two figures are similar if they have the same shape. All their linear measurements (sides, diagonals, heights) are multiplied by the same number, the scale factor $k$:

$$k = \frac{\text{measurement of the new figure}}{\text{measurement of the original figure}}$$

If $k > 1$ it is an enlargement and if $k < 1$ it is a reduction. On a plan at a scale of $1:200$, every $1$ cm in the drawing is $200$ cm in real life.

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A graphic designer has a rectangular photograph $10 \text{ cm}$ wide and $15 \text{ cm}$ high. If they decide to enlarge it to make a poster so that the new width is…

A graphic designer has a rectangular photograph $10 \text{ cm}$ wide and $15 \text{ cm}$ high. If they decide to enlarge it to make a poster so that the new width is $40 \text{ cm}$, what will the height of the poster be if both shapes must be similar?

10 cm 15 cm 40 cm x
Solve
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On the map of a housing development, the scale used is $1:500$. If the community swimming pool measures $3 \text{ cm}$ long on the map, what is its actual length in…

On the map of a housing development, the scale used is $1:500$. If the community swimming pool measures $3 \text{ cm}$ long on the map, what is its actual length in meters?

Map 3 cm Scale 1:500 Actual size ? m
Solve

Shadows and similar triangles

If the sun (or a lamp) lights an object and a person at the same time, both form right triangles with the same slant of the rays. Those triangles are similar, so their sides are proportional. This is the idea behind Thales' theorem.

Two similar right triangles formed by a pole and a tree with their shadows on the ground. pole tree shadow shadow
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At a certain time of day, a vertical post $2 \text{ m}$ high casts a shadow of $1.5 \text{ m}$. At that exact same moment, the shadow of a nearby electricity pylon…

At a certain time of day, a vertical post $2 \text{ m}$ high casts a shadow of $1.5 \text{ m}$. At that exact same moment, the shadow of a nearby electricity pylon measures $12 \text{ m}$. Applying Thales' theorem, calculate the height of the pylon.

h 2m 1,5m 12m
Solve
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A group of 2nd ESO students wants to calculate the height of the basketball hoop in the playground without using a ladder. At a given moment, Luis, who is…

A group of 2nd ESO students wants to calculate the height of the basketball hoop in the playground without using a ladder. At a given moment, Luis, who is $1.60 \text{ m}$ tall, casts a shadow of $0.80 \text{ m}$. If at that exact same moment the basketball hoop's shadow is $1.52 \text{ m}$ long, what is the height of the basketball hoop? Draw a diagram of the situation identifying the similar triangles.

h = ? 1.52 m Basketball hoop shadow 1.60 m 0.80 m Luis's shadow

You can do it on paper and send a photo with your answer.

Solve
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In a town square, there is a vertical flagpole. At a distance of $12\text{ m}$ from its base, there is a spotlight on the ground projecting light. A teenager measuring…

In a town square, there is a vertical flagpole. At a distance of $12\text{ m}$ from its base, there is a spotlight on the ground projecting light. A teenager measuring $1.65\text{ m}$ in height stands between the spotlight and the flagpole, at a distance of $3\text{ m}$ from the spotlight.

  1. a) Calculate the height of the flagpole knowing that the shadow of the boy's head coincides exactly with the top of the flagpole.
  2. b) If the boy walks $2\text{ m}$ towards the flagpole, how long will his shadow cast on the flagpole itself be?
Solve

Slopes and triangles inside triangles

When a line parallel to one side cuts a triangle, a smaller triangle similar to the large one appears. It is useful for a shelf in a bookcase, a sloped roof, or an antenna at another scale. If two structures are similar, all their measurements share the same scale factor.

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A cell phone tower $12 \text{ m}$ high is anchored to the ground with two guy wires extending from its top end. Each wire is anchored at a distance of $5 \text{ m}$ from…

A cell phone tower $12 \text{ m}$ high is anchored to the ground with two guy wires extending from its top end. Each wire is anchored at a distance of $5 \text{ m}$ from the base of the tower.

  1. What is the total length of wire needed to secure the tower?

  2. If we want to set up another similar tower with a height of $18 \text{ m}$, at what distance from the base should we anchor the wires so that the structure remains proportional?

Solve
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A carpenter is designing a bookcase shaped like a right-angled triangle. He has placed an intermediate shelf parallel to the base. The total height of the bookcase is…

A carpenter is designing a bookcase shaped like a right-angled triangle. He has placed an intermediate shelf parallel to the base. The total height of the bookcase is $150 \text{ cm}$ and the base measures $90 \text{ cm}$. If the intermediate shelf is located at a height of $60 \text{ cm}$ from the base:

  1. What is the length of this intermediate shelf?

  2. What is the length of the sloped wooden beam forming the hypotenuse of the full bookcase?

Solve
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A rectangular solar panel is to be installed on a sloping roof. The roof has a pitch such that for every $4\text{ m}$ of horizontal distance, it rises by $3\text{ m}$ in…

A rectangular solar panel is to be installed on a sloping roof. The roof has a pitch such that for every $4\text{ m}$ of horizontal distance, it rises by $3\text{ m}$ in height.

  1. a) Calculate the actual length of the section of roof that covers a horizontal distance of $8\text{ m}$.
  2. b) If the solar panel is $2\text{ m}$ long and is placed along the slope of the roof, what vertical height $h$ does the panel span between its bottom and top edges? (Use similarity of triangles).
4 m 3 m h 2 m 8 m L = ?
Solve

Areas and volumes in similar figures

If the scale factor of the lengths is $k$, areas change by $k^2$ and volumes by $k^3$. This makes sense: an area has two dimensions and a volume has three.

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Two squares are similar. The side of the first one measures $4 \text{ cm}$ and the side of the second one measures $12 \text{ cm}$. Calculate the similarity ratio $k$…

Two squares are similar. The side of the first one measures $4 \text{ cm}$ and the side of the second one measures $12 \text{ cm}$. Calculate the similarity ratio $k$ and explain how many times larger the area of the second square is compared to the first.

Two similar squares: the first with side 4 cm and the second with side 12 cm, joined by a similarity arrow A₁ L₁ = 4 cm A₂ L₂ = 12 cm k = ?
Solve
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On a floor plan of a room drawn to a scale of $1:50$, the dimensions of a rectangular desk are $2.4 \text{ cm}$ wide and $3.6 \text{ cm}$ long. 1. Calculate the actual…

On a floor plan of a room drawn to a scale of $1:50$, the dimensions of a rectangular desk are $2.4 \text{ cm}$ wide and $3.6 \text{ cm}$ long.

  1. Calculate the actual dimensions of the desk in meters.

  2. If the actual area of the room is $12 \text{ m}^2$, what is the area occupied by the room on the floor plan expressed in $\text{cm}^2$?

Solve
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A scale model of a monument has been made using a scale of $1:250$. If the actual monument has a volume of $1500 \text{ m}^3$, calculate the volume of the model in cubic…

A scale model of a monument has been made using a scale of $1:250$. If the actual monument has a volume of $1500 \text{ m}^3$, calculate the volume of the model in cubic decimeters ($\text{dm}^3$). Remember the relationship between the scale factor and the ratio of the volumes.

Solve

Two right triangles at once

Sometimes there are two right triangles that share a leg. We don't know that leg, but we do know the two hypotenuses. The idea is to write Pythagoras for each triangle and subtract the equations: the unknown term that repeats cancels out.

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A lighthouse $35\text{ m}$ high is situated on a vertical cliff. From a boat at sea, it is observed that the distance to the base of the lighthouse (the point where the…

A lighthouse $35\text{ m}$ high is situated on a vertical cliff. From a boat at sea, it is observed that the distance to the base of the lighthouse (the point where the lighthouse meets the cliff) is $100\text{ m}$ and the distance to the top of the lighthouse is $110\text{ m}$. Calculate:

  1. a) The height of the cliff above sea level.
  2. b) The horizontal distance from the boat to the base of the cliff.
Solve

The altitude theorem

In a right triangle, the altitude drawn to the hypotenuse divides it into two segments, $m$ and $n$. It forms two smaller triangles that are similar to each other (and to the large one).

Right triangle with the right angle at the top and the altitude to the hypotenuse, which divides it into two segments m and n. m n h

In the figure the hypotenuse is the horizontal base and the right angle is at the top vertex. By similarity of the two small triangles, $\dfrac{h}{m} = \dfrac{n}{h}$. The result follows from this.

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A courier company uses drones to deliver packages. To ensure a safe landing on a rooftop, the drone projects two laser beams that form a right angle at the emitter. The…

A courier company uses drones to deliver packages. To ensure a safe landing on a rooftop, the drone projects two laser beams that form a right angle at the emitter. The drone is at a height $h$ above the rooftop. The laser impact points on the ground are aligned with the vertical below the drone and are $15.6\text{ m}$ apart. If one of the impact points is $3.9\text{ m}$ from the vertical below the drone, calculate:

  1. a) The height $h$ at which the drone is flying using the altitude theorem.
  2. b) The distance from the drone to each of the two landing points (the legs of the triangle).
Drone h d₁ d₂ 3.9 m 15.6 m
Solve

Putting it all together: pyramids, rhombuses, and signs

The hardest problems combine several ideas. It is always a good idea to: (1) draw a picture, (2) find the right or similar triangles, (3) decide whether the scale factor applies to lengths, areas, or volumes.

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For a film festival, a scale model of a regular square-based pyramid has been built. The actual pyramid has a base side length of $24\text{ m}$ and a height of…

For a film festival, a scale model of a regular square-based pyramid has been built. The actual pyramid has a base side length of $24\text{ m}$ and a height of $16\text{ m}$. If the model was built with a scale factor of $k = 1:80$, calculate:

  1. a) The length of the slant edge of the actual pyramid (you will need to apply the Pythagorean theorem twice).
  2. b) The volume of the model in cubic centimeters, knowing that the volume of a pyramid is $V = \frac{1}{3} \times \text{Base area} \times \text{Height}$. (Use the relationship $k^3$ for the volume).
Solve
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A road sign is in the shape of an equilateral triangle with a side length of $90\text{ cm}$. Inside it, a red border forms another similar equilateral triangle. If the…

A road sign is in the shape of an equilateral triangle with a side length of $90\text{ cm}$. Inside it, a red border forms another similar equilateral triangle. If the area of the white section (the inner triangle) is exactly half of the total area of the sign, what is the scale factor between the smaller and larger triangles? How long is the side of the inner triangle? Round to two decimal places.

Inner area = A / 2 x 90 cm
Solve
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The logo for a new clothing brand is a rhombus inscribed inside a rectangle measuring $16\text{ cm}$ long and $12\text{ cm}$ wide. The vertices of the rhombus are the…

The logo for a new clothing brand is a rhombus inscribed inside a rectangle measuring $16\text{ cm}$ long and $12\text{ cm}$ wide. The vertices of the rhombus are the midpoints of the sides of the rectangle. Calculate:

  1. a) The perimeter of the rhombus.
  2. b) The area of a second similar logo whose perimeter is triple the first. Justify how the area changes without calculating the new side lengths.
16 cm 12 cm 8 6 l
Solve

Summary

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