In this explainer, we will learn how to use the Pythagorean theorem to solve
problems in three dimensions.
We recall that the Pythagorean theorem describes the relationship between the
lengths of the three sides of a right triangle, and we should already be familiar
with applying this result to problems in two dimensions.
Theorem: The Pythagorean Theorem in Two Dimensions
In a right triangle, the square of the hypotenuse is equal to the sum of the
squares of the two shorter sides. For the triangle below,
The Pythagorean theorem can be applied to three-dimensional problems in a number of
ways. One method is to use right triangles contained within the faces of the
three-dimensional object or to take two-dimensional βslicesβ through
its interior. To identify such triangles, we need to be familiar with the parts and
properties of some common three-dimensional objects, such as pyramids and cones.
We recall first two special types of pyramids. First, a right pyramid is a pyramid
whose apex is vertically above the center, or in the case of a triangle, the
centroid, of its base. Second, a regular pyramid is a right pyramid whose base is a
regular polygon.
Key Terms: Parts of a Pyramid
Consider the regular square pyramid in the figure below. We recall the terminology
used to describe the lengths and parts of a pyramid. The perpendicular height of a
pyramid is the perpendicular distance between the apex of the pyramid and
its base, and it is perpendicular to any straight line it intersects in the
pyramidβs base. The slant height of a pyramid is the perpendicular distance
from one of the sides of the base to the apex of the pyramid. We can note that the
slant height will be the same in the four lateral faces.
Key Terms: Parts of a Cone
In the figure below, we recall the terminology used to describe the lengths
and parts of a cone. We note that the main lengths in a cone are the slant height,
the perpendicular height, and the base radius of the cone. These three lengths
form a right triangle.
In the course of this explainer, we will see examples of how we can apply the
Pythagorean theorem to right triangles within both pyramids and cones to calculate
unknown lengths. In our first example, however, we will consider how to find the
area of a diagonal shape contained within a cube, given the
cubeβs edge length.
Example 1: Finding the Area of a Diagonal Shape in a Cube given Its
Dimensions
Given that is a cube whose edge length is
cm
and is the midpoint of
, find the area of the rectangle
.
Answer
We recall first that the area of a rectangle is found using the formula
. For rectangle
, the area will therefore be equal to
.
The length of is
cm
as it is parallel to the edge of the cube and has the same length. To find the length
of consider the square base of the cube. As is the midpoint of ,
it divides into two equal parts, each of
length . As all of the interior angles of a square are
, the triangle formed by
, ,
and is a right triangle at
.
The hypotenuse of this triangle is and so,
by the Pythagorean theorem,
Substituting for
and for gives
We solve this equation by first evaluating each square and then simplifying:
Taking the square root of each side of the equation gives
The area of rectangle is therefore equal to
We now consider two examples relating to pyramids. In the first example,
we calculate the height of a pyramid whose base is an equilateral triangle, given the
triangleβs side length and the length of the pyramidβs lateral
edge.
Example 2: Finding the Height of a Pyramid by Applying the Pythagorean
Theorem
is a regular pyramid whose base
is an equilateral triangle whose side length is
32 cm. If the length of its lateral edge is
88 cm,
find the height of the pyramid to the nearest hundredth.
Answer
We begin by sketching the pyramid as shown below (not to scale).
We are asked to calculate the height of the pyramid. This is the vertical
distance from its vertex to the center of the base of the
pyramid, which we will label . We draw this height on our
diagram and also the line connecting one vertex of the base to point
.
is the lateral edge of this pyramid,
the length of which is given in the question. is a line segment on the base of the
pyramid, and is the vertical height
of the pyramid, which is perpendicular to
. Hence, we have a right triangle formed
by , ,
and . By the Pythagorean theorem,
The length of is given in the
question. In order to calculate the length of ,
it is first required that we calculate the length of
.
Now consider the equilateral triangular base of the pyramid. Point
is the centroid of this triangle, the point at which the three
medians of the triangle intersect. Point is the midpoint of
side .
As triangle is equilateral, its three medians are all of
equal length. We also know that the centroid of a triangle divides each median in
the ratio from the vertex. Hence,
and
.
Now consider triangle . This is a right triangle at
as the median
divides the equilateral triangle
into two congruent right triangles. The length of
is half the length of
, so it is
16 cm. We can therefore
apply the Pythagorean theorem to calculate the length of
, which will then enable us to calculate
the length of .
Applying the Pythagorean theorem in triangle , we have
Substituting and
gives
We solve for by square rooting:
We can then calculate the length of
by recalling that :
Finally, we return to our application of the Pythagorean theorem in triangle
,
Substituting and
gives
Subtracting
from each side of the equation and evaluating the squares gives
We solve for by square rooting:
The height of the pyramid, to the nearest hundredth, is
86.04 cm.
We now consider a problem in which we are given the net of a pyramid and are
required to calculate its vertical height and its slant height. We will need to
interpret the net carefully to understand which of the pyramidβs dimensions
have been given.
Example 3: Finding the Vertical Height and Slant Height of a Square Pyramid given the
Dimensions of Its Net
Consider the net of a square pyramid with the shown dimensions.
Find, to the nearest hundredth, the pyramidβs height.
Find, to the nearest hundredth, the pyramidβs slant height.
Answer
Part 1
Let us begin by sketching the pyramid in three dimensions. From the net, we
identify that the side length of the square base is
5 cm. The lines of
length 4 cm
represent the other two sides of the pyramidβs triangular faces, and
so we deduce that the length of the pyramidβs lateral edge is
4 cm.
Point on our sketch is the point at which the vertical
line drawn from the apex of the pyramid
meets the pyramidβs square base. This point is in the center of the
base. The vertical height of the pyramid is
, which is perpendicular to any line
in the pyramidβs base passing through point
. In particular, it is perpendicular to the line segments
joining point to each of the pyramidβs vertices. Hence, the triangle formed by the vertical height
, the lateral edge
, and
is a right triangle at
, as shown below.
Hence, by the Pythagorean theorem,
The length of is given, but it is
necessary that we calculate the length of
before we can calculate the length of
. Consider the square base of the
pyramid, and recall that lies in the center of this square. We also recall that the center of a square is the intersection point of the two
diagonals, and both diagonals bisect each other.
The length of is half the length of
diagonal . This diagonal divides the
square base into two congruent right triangles. Hence, by the Pythagorean
theorem in triangle ,
Substituting 5 for each of and
gives
We solve for by square rooting:
The length of is half this value:
Returning to our earlier statement of the Pythagorean theorem in triangle
, we substitute
for
and 4 for to give the following
equation:
This equation can be solved by first evaluating the squares and then
isolating :
Finally, we solve for by finding the square root of
each side of this equation:
The vertical height of the pyramid, to the nearest hundredth, is
1.87 cm.
Part 2
We now draw in the slant height of the pyramid along one of its faces. We define to be the midpoint of side
.
The slant height forms a right triangle with the perpendicular height of the
pyramid and the line segment connecting points
and , that is, the line segment connecting the center of
the base with the midpoint of one of the sides of the base. Hence, by the
Pythagorean theorem,
In the first part of the question, we determined the value of
to be . is parallel to side
of the pyramidβs base,
and as is the center of the base,
. Hence, the length of
is
cm. Substituting these values into our statement of the Pythagorean theorem in triangle
gives
Evaluating and simplifying gives
To solve for , we find the square root of each side of
this equation:
The slant height of the pyramid, to the nearest hundredth, is
3.12 cm.
We now consider problems related to cones. In our next example, we will
calculate the circumference and base area of a right circular cone by first
finding its radius. Recall that, in such a cone, the relationship between the base
radius, the perpendicular height, and the slant height can be described using the
Pythagorean theorem.
Example 4: Finding the Circumference and Area of the Base of a Right Circular Cone
given Its Heights by Applying the Pythagorean Theorem
A right circular cone has height
90 cm
and slant height 106 cm. Find the circumference and area of the base in terms of
.
Answer
We recall first the formulas for calculating the circumference and area
of a circle:
and , where
represents the radius. Hence, we are required to first calculate the radius of
the coneβs circular base.
We now sketch the cone. Recall that the height of a right circular cone,
often denoted by , is the perpendicular distance between
the center of the base and the apex of the cone. The slant height, denoted
by , is the distance from any point on the circumference
of the circular base to the apex, along the surface of the cone.
Next, we note that if we draw the radius of the cone, the triangle formed
by the radius, the height, and the slant height of the cone is a right
triangle.
Hence, by the Pythagorean theorem,
Subtracting from each side of the equation and
simplifying give
We solve for by taking the positive square root:
Substituting into the formula for the circumference
of a circle gives
The question specifies that our answers should be given in terms of
, so we are not required to evaluate this.
Substituting the value of from our penultimate
stage of working into the formula for the area of a circle gives
Hence, the circumference of the circular base is
cm
and the area is
cm2.
Example 5: Using the Pythagorean Theorem to Calculate the Height of a Cone given
Its Net
A piece of paper in the shape of a circular sector having a radius of
72 cm
and an angle of
is folded in a way so that the points
and meet to form a circular cone of the greatest possible
area. Determine the coneβs height to the nearest hundredth.
Answer
The given figure represents the net of the cone. We will begin by sketching
the cone in three dimensions. The slant height of the folded cone will be
equivalent to the length of the line segments joining each of points
and to the center of the circular
sector. This is the length of the radius of the sector, which is given in the
question as 72 cm. If the cone is to have the greatest possible area, then the paper must not
overlap, and so the circumference of the circle at the base of the cone must
be equal to the arc length of the circular sector.
We recall that the base radius, perpendicular height, and slant height of a
cone form a right triangle. Hence, by applying the Pythagorean theorem to
this right triangle in the cone above, we have
To determine the height of the cone, we must first calculate its base
radius. We will begin by calculating the arc length of the circular sector,
which is equal to the circumference of the circular base. Recall the formula
where is the central angle of the sector, measured in
degrees,
and is the radius. Note that we have used a capital
here to distinguish this length from the radius of the
coneβs base. We can then substitute
and to calculate the arc length:
As the arc length of the circular sector is equal to the circumference
of the coneβs circular base, we can calculate the radius of the circle
by recalling the formula
. Hence,
Dividing both sides of this equation by gives
Finally, we substitute this value of into our earlier
statement of the Pythagorean theorem:
Evaluating the squares gives
To solve for , we first subtract 3βββ025 from each side of
the equation and then find the square root:
The height of the cone, to the nearest hundredth, is
46.47 cm.
In the previous problems, we have seen how we can take two-dimensional
βslicesβ through the interior of a three-dimensional object in order
to create right triangles in which we can apply the Pythagorean theorem. An alternative way in which the Pythagorean theorem can be applied to
three-dimensional problems is in a three-dimensional extension of the theorem
itself. We will demonstrate this for the case of calculating the length of the
diagonal of a cuboid.
First, we consider more specifically what is meant by the diagonal of a cuboid. We must be careful to distinguish the interior diagonals of a cuboid from its face
diagonals, those that lie within its two-dimensional faces. For example, in the case
of the cuboid below, the line segments
,
,
and are each face diagonals of the cuboid.
In fact, a cuboid has 12 face diagonals in total: 2 in each of its 6 faces.
The diagonals, also sometimes called the space or interior diagonals of a
cuboid, are different. These are the line segments that connect opposite
vertices of the cuboid and pass through its interior. For example, in the cuboid
above, one of the cuboidβs diagonals is
. In fact, a cuboid has four diagonals,
which for the cuboid above are ,
,
, and . These four line segments are congruent and all intersect at a common point at the
center of the cuboid, as shown in the figure below.
We now consider how to calculate the length of the diagonal of a cuboid. Consider a cuboid with sides of lengths
, , and units,
respectively, as shown below. Suppose we wish to calculate the length of the
cuboidβs diagonal, .
We can achieve this using two iterations of the Pythagorean theorem. We begin
by drawing the base diagonal of the cuboid, which we will label its length as .
By applying the Pythagorean theorem in the right triangle
on the base of the cuboid, in which
is the hypotenuse, we have
To calculate the length of , we can apply the Pythagorean
theorem again in the right triangle . Side
is the hypotenuse of this triangle and the two shorter sides
are and , so we have
Substituting the expression for obtained from our first
application of the Pythagorean theorem gives
This is the three-dimensional extension of the Pythagorean theorem.
Theorem: The Three-Dimensional Extension of the Pythagorean Theorem
In a cuboid with side lengths , ,
and and interior diagonal
as shown below,
We now demonstrate the application of the Pythagorean theorem in three dimensions
to calculate the length of the diagonal of a cuboid given its length, width,
and depth.
Example 6: Finding the Length of the Diagonal of a Cuboid Using the Pythagorean
Theorem
Find the length of the diagonal of a rectangular prism of sides
3 cm,
4 cm, and
6 cm. Round your answer to the nearest hundredth.
Answer
Although not essential, we begin by sketching the cuboid.
The diagonals of this cuboid are ,
,
,
and . These four line segments each connect opposite vertices of the cuboid and pass
through its interior. The diagonals of a cuboid are of equal length, so it is
entirely arbitrary which we choose to calculate. Let us choose
and add this line segment to our
diagram.
The three-dimensional extension of the Pythagorean theorem states that
where is the diagonal of a cuboid and
, , and
are its three side lengths. Here , ,
and are
3 cm,
4 cm,
and 6 cm,
and the diagonal is , so we have
Simplifying gives
We solve for by finding the square root, ignoring the
negative solution as is a length:
The length of the diagonal of the rectangular prism, to the nearest
hundredth, is
7.81 cm.
In the previous problem, we chose to apply the three-dimensional version of the
Pythagorean theorem. It would also have been possible to apply two iterations of the
Pythagorean theorem in two dimensions. If we had drawn the base diagonal
, then applying the Pythagorean theorem in
the right triangle , in which
is the hypotenuse, would give
Then, applying the Pythagorean theorem a second time in triangle
, in which is the
hypotenuse, would give
We see that this is consistent with the penultimate stage of working in our
previous method. Both methods of course yield the same result, but applying the
three-dimensional extension of the Pythagorean theorem is more efficient.
Let us finish by recapping some key points from this explainer.
Key Points
The two-dimensional Pythagorean theorem can be applied to right triangles
within the faces of a three-dimensional object, or to two-dimensional slices
through its interior, in order to calculate unknown lengths.
The perpendicular height of a regular pyramid is perpendicular to any
straight line passing through the center of its base and lying on this base,
and hence a right triangle can be formed with the lateral edge. The slant
height of a regular pyramid is the perpendicular distance from any side of
the base to the pyramidβs apex.
The base radius , vertical height
, and slant height
of a cone form a right triangle and are
hence related by the Pythagorean theorem as follows:
The extension of the Pythagorean theorem to three dimensions states that for a
cuboid with side lengths , ,
and and a diagonal of length ,
as shown below,
This is analogous to applying two iterations of the Pythagorean theorem in two
dimensions.
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