ACT Math : Geometry

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : How To Find The Length Of An Edge Of A Tetrahedron

A tetrahedron has a volume that is twice the surface area times the edge. What is the length of the edge? (In the answer choices,  represents edge.)

Possible Answers:

Correct answer:

Explanation:

The problem states that the volume is:

The point of the problem is to solve for the length of the edge. Becasuse there are no numbers, the final answer will be an expression. 

In order to solve for it, we will have to rearrange the formula for volume in terms of

Example Question #6 : Tetrahedrons

Calculate the diagonal of a regular tetrahedron (all of the faces are equilateral triangles) with side length .

Possible Answers:

Correct answer:

Explanation:

The diagonal of a shape is simply the length from a vertex to the center of the face or vertex opposite to it. With a regular tetrahedron, we have a face opposite to the vertex, and this basically amounts to calculating the height of our shape.

We know that the height of a tetrahedron is  where s is the side length, so we can put  into this formula:

which gives us the correct answer.

Example Question #1 : How To Find The Surface Area Of A Tetrahedron

If the edge length of a tetrahedron is , what is the surface area of the tetrahedron?

Possible Answers:

Correct answer:

Explanation:

Write the formula for finding the surface area of a tetrahedron.

Substitute the edge and solve.

Example Question #81 : Solid Geometry

Each of the faces of a regular tetrahedron has a base of  and a height of . What is the surface area of this tetrahedron?

Possible Answers:

Correct answer:

Explanation:

The surface area is the area of all of the faces of the tetrahedron. To begin, we must find the area of one of the faces. Because a tetrahedron is made up of triangles, we simply plug the given values for base and height into the formula for the area of a triangle:

Therefore, the area of one of the faces of the tetrahedron is . However, because a tetrahedron has 4 faces, in order to find the surface area, we must multiply this number by 4:

Therefore, the surface area of the tetrahedron is .

Example Question #1 : How To Find The Surface Area Of A Tetrahedron

What is the surface area of a regular tetrahedron with a slant height of ?

Possible Answers:

Cannot be determined

Correct answer:

Explanation:

If this is a regular tetrahedron, then all four triangles are equilateral triangles. 

If the slant height is , then that equates to the height of any of the triangles being .

In order to solve for the surface area, we can use the formula

where  in this case is the measure of the edge.

The problem has not given the edge; however, it has provided information that will allow us to solve for the edge and therefore the surface area. 

Picture an equilateral triangle with a height .
Tetrahedron

 

Drawing in the height will divide the equilateral triangle into two 30/60/90 right triangles. Because this is an equilateral triangle, we can deduce that finding the measure of the hypotenuse will suffice to solve for the edge length (). 

In order to solve for the hypotenuse of one of the right triangles, either trig functions or the rules of the special 30/60/90 triangle can be used. 

Using trig functions, one option is using .

Rearranging the equation to solve for

Now that  has been solved for, it can be substituted into the surface area equation.

Example Question #1 : How To Find The Surface Area Of A Tetrahedron

What is the surface area of a regular tetrahedron when its volume is 27?

Possible Answers:

Correct answer:

Explanation:

The problem is essentially asking us to go from a three-dimensional measurement to a two-dimensional one. In order to approach the problem, it's helpful to see how volume and surface area are related. 

This can be done by comparing the formulas for surface area and volume:

 

We can see that both calculation revolve around the edge length.

That means, if we can solve for  (edge length) using volume, we can solve for the surface area. 

Now that we know , we can substitute this value in for the surface area formula:

Example Question #81 : Solid Geometry

Find the volume of a regular tetrahedron if one of its edges is  long.

Possible Answers:

Correct answer:

Explanation:

Write the volume equation for a tetrahedron.

In this formula,  stands for the tetrahedron's volume and  stands for the length of one of its edges.

Substitute the given edge length and solve.

Rationalize the denominator.

Example Question #82 : Solid Geometry

Find the volume of a tetrahedron if the side length is .

Possible Answers:

Correct answer:

Explanation:

Write the equation to find the volume of a tetrahedron.

Substitute the side length and solve for the volume.

Rationalize the denominator.

Example Question #82 : Solid Geometry

What is the volume of a regular tetrahedron with an edge length of 6?

Possible Answers:

Correct answer:

Explanation:

The volume of a tetrahedron can be solved for by using the equation:

where  is the measurement of the edge of the tetrahedron. 

This problem can be quickly solved by substituting 6 in for

Example Question #1 : How To Find The Radius Of A Sphere

The surface area of a sphere is  feet. What is the radius?

Possible Answers:

Correct answer:

Explanation:

Solve the equaiton for the surface area of a sphere for the radius and plug in the values:

 

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