Missing Measurements of Solids from the Area

Concept sheet | Mathematics

In some problems, the solid’s area and all of its measurements, except one, are given. In these cases, it is necessary to know how to find this missing measurement.

The procedure for finding a solid’s missing measurement is generally the same regardless of the type of solid. The following are the main steps.

Rule
  1. Identify the measurements given.

  2. Determine the formula to use.

  3. Replace the variables with the given measurements.

  4. Isolate the sought variable.

Finding a Missing Measurement of a Cube

To find the measure of the side of a cube from its area, refer to the appropriate area formula (i.e., total area, lateral area or area of ​​the bases). Next, perform inverse operations to find the missing measurement.

Example

A toy company wants to replace some of the sides of their cardboard boxes with a thick transparent material to enable customers to see the packaged products inside. To order the material, the company needs to know the box’s dimensions. Given that the total area of the box is |2\ 646\ \text{cm}^2,| determine the measure of the box’s edges.

Cubic box with a missing side measurement.

See solution

Finding a Missing Measurement of a Prism

To determine the missing measure of a prism from its area, refer to the appropriate area formula (i.e., total area, lateral area or area of ​​the bases). Next, perform inverse operations to determine the missing measurement.

Example

Before making a delivery, a trucker must ensure that the dimensions of the object being transported does not exceed the width of the trailer. He only has access to the measurements identified in the following image for his next delivery. Help him determine the measure of the box’s width.

Cardboard box in the shape of a rectangular prism with an unknown width.

See solution

Finding a Missing Measurement: Prisms

Finding a Missing Measurement: Prisms

Moments in the video:

  • 00:00-Introduction
  • 00:14-Steps to find a missing measurement
  • 00:32-Example of finding the width of prism
  • 04:08-Example of finding the measurement of the side of a cube
  • 05:33-Conclusion

Finding a Missing Measurement of a Cylinder

To find a missing measure of a cylinder from its area, refer to the appropriate area formula (i.e., total area, lateral area or area of ​​the bases). Next, perform inverse operations to find the missing measurement.

Example

Sergei wants to coat his new lidded, cylindrical trash can with an anti-rust coating. His neighbour tells him about a product that covered his |1{.}335\ \text{m}^2| garbage can with a single coat. However, Sergei's trash can is twice as tall as his neighbour's. If the diameter of his neighbour’s garbage is |0{.}5\ \text{m}|, how tall is Sergei's trash can?

Cylindrical trash can with an unknown height.

See solution

Example

What is the measure of a tennis ball’s radius if a cylindrical container that can hold exactly 3 balls has a lateral area of |379{.}84\ \text{cm}^2|?

See solution

Finding a Missing Measurement: Cylinders

Finding a Missing Measurement: Cylinders

Moments in the video:

  • 00:00-Introduction
  • 00:12-Steps to find a missing measurement
  • 00:26-Example of finding the height of a cylinder
  • 00:33-Identifying the given measurements
  • 00:45-Determining the formula to use
  • 01:16-Replacing the variables with the given measurements
  • 01:50-Isolating the missing variable
  • 02:40-Conclusion

Finding a Missing Measurement of a Pyramid

To find a missing measurement of a pyramid from the area, refer to the appropriate area formula (i.e., total area, lateral area or area of ​​the bases). Next, perform inverse operations to find the missing measurement.

Finding a Measurement of the Base of a Pyramid

Example

Gabriela wants to buy a decorative incense burner for her living room. The burner is a regular octagonal pyramid. Gabriela doesn't know if it can fit on a shelf |15\ \text{cm}| deep.

Gabriela knows the total area of the incense burner is |789{.}12\ \text{cm}^2.| The apothem of the pyramid is |15\ \text{cm}| and the measure of one side of the octagon is |8\ \text{cm}.|

Can the incense burner fit on the shelf if the base’s apothem |(\color{#3B87CD}{a_b})| is perpendicular to the wall?

Incense burner in the shape of a regular octagonal pyramid with an unknown base apothem.

See solution

Be careful!

Since a regular pyramid is, in fact, a pyramid with a regular polygon as its base, be careful not to confuse the base’s apothem with that of the pyramid.

The apothem is usually identified by the variable |a.| To differentiate between the two apothems, add a subscript. Thus, the pyramid’s apothem becomes |a_p| and the base’s apothem becomes |a_b.| The choice of the subscript or the way of identifying the 2 measures may vary in different contexts.

Finding the Measure of the Apothem of a Pyramid

Example

Stefan plays piano and he frequently uses an old metronome that is shaped like a square-based pyramid.

To polish the metronome, he needs to know the measure of the pyramid’s apothem. He knows that the base of the metronome measures |10\ \text{cm}| and the total area of the pyramid is |512{.}4\ \text{cm}^2.|

What is the measure of the metronome’s apothem?

Metronome in the shape of a square-based pyramid where the measure of the apothem is missing.

See solution

Finding a Missing Measurement: Pyramids

Finding a Missing Measurement: Pyramids

Moments in the video:

  • 00:00-Introduction
  • 00:13-Steps to Find a Missing Measurement
  • 00:31-Example 1: Apothem of a Square-Based Pyramid
  • 03:03-Example 2: Perimeter of a Tetrahedron
  • 05:40-Conclusion

Finding a Missing Measurement of a Cone

To find the missing measurement of a cone from its area, refer to the appropriate area formula (i.e., total area, lateral area, or area of ​​the bases). Next, perform inverse operations to find the desired measurement.

Finding the Measure of the Base of a Cone

Example

Parents bought cone-shaped party hats to distribute to all the guests at their child’s birthday party. To ensure that the model found online is large enough for the children's heads, they need to know the measure of the circle’s radius at the base of the hat.

Conical party hat with a missing radius measurement.

See solution

Finding the Measurement of the Apothem of a Cone

Since pyramids and cones have a similar shape, the procedure for finding the measurement of the cone’s apothem is the same as that of a pyramid.

Example

A resort wants to install a ski lift to provide a panoramic view. To complete this project, the resort needs to know the distance between the foot of the mountain and the summit.

Using topographic maps of the region, the lateral area of ​​the conical-shaped mountain is estimated to be |1{.}7\ \text{km}^2| and the measurement of the radius of the base is about |500\ \text{m}.| What should be the ski lift’s length, to the nearest metre?

Conical mountain with a ski lift.

See solution

Be careful!

Sometimes, the height of a pyramid or cone must be determined from the area even though it is not in the formula for the area. In that case, find the apothem of the solid or the radius of the base. Then, use the Pythagorean Theorem to calculate the height.

Finding the Measure of the Height of a Cone or Pyramid

Whether for a pyramid or a cone, the procedure is very similar.

Example

During kickoffs in football games, kickers often use small cone-shaped stands to hold the ball upright.

If the tip of the ball coincides with the apex of the cone, how far is the ball from the ground when it is on the stand?

Conical football stand with missing height measurement.

See solution

The previous example demonstrates that it is usually necessary to find the measure  of the apothem before determining the height using the Pythagorean Theorem. Finding the height of a cone or pyramid from its area requires a few more calculations than simply calculating the measure of the apothem.

Be careful!

If, instead of the lateral area, we are given the volume of a cone or a pyramid and the dimensions of the base, the height can be found directly. This measurement is in the formula for the volume of cones and pyramids. The formula for the volume of these solids is |V= \dfrac{A_b \times h}{3}.|

To find the apothem of one of these solids from the volume, first find the height. Then, use the Pythagorean Theorem to calculate the apothem. Read the concept sheet about finding the measurement of the apothem of a pyramid or a cone for more information.

Finding a Missing Measurement of a Sphere

To calculate the radius of a sphere when its area is known, use the formula for the area of a sphere and perform inverse operations to isolate the radius.

Example

What is the height of a traditional billiard ball (in millimetres)?

Billiard ball with a missing height measurement.

See solution

Exercise

Exercise

Finding a Missing Measurement From the Area of a Solid

Mathematics Secondary2-3