# ISEE Upper Level Math : Tetrahedrons

## Example Questions

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### Example Question #1 : Tetrahedrons

A triangular pyramid, or tetrahedron, with volume 100 has four vertices with Cartesian coordinates

where .

Evaluate

Explanation:

The tetrahedron is as follows (figure not to scale):

This is a triangular pyramid with a right triangle with legs 10 and  as its base; the area of the base is

The height of the pyramid is 5, so

Set this equal to 100 to get :

### Example Question #2 : Tetrahedrons

A triangular pyramid, or tetrahedron, with volume 1,000 has four vertices with Cartesian coordinates

where .

Evaluate

Explanation:

The tetrahedron is as follows:

This is a triangular pyramid with a right triangle with two legs of measure  as its base; the area of the base is

Since the height of the pyramid is also , the volume is

.

Set this equal to 1,000:

### Example Question #3 : Tetrahedrons

A triangular pyramid, or tetrahedron, with volume 240 has four vertices with Cartesian coordinates

where .

Evaluate

Explanation:

The tetrahedron is as follows (figure not to scale):

This is a triangular pyramid with a right triangle with two legs of measure  as its base; the area of the base is

The height of the pyramid is 24, so the volume is

Set this equal to 240 to get :

### Example Question #4 : Tetrahedrons

In three-dimensional space, the four vertices of a tetrahedron - a solid with four faces - have Cartesian coordinates .

Give its volume.

Explanation:

A tetrahedron is a triangular pyramid and can be looked at as such.

Three of the vertices -  - are on the -plane, and can be seen as the vertices of the triangular base. This triangle, as seen below, is isosceles:

Its base and height are both 18, so its area is

The fourth vertex is off the -plane; its perpendicular distance to the aforementioned face is its -coordinate, 9, so this is the height of the pyramid. The volume of the pyramid is

### Example Question #5 : Tetrahedrons

In three-dimensional space, the four vertices of a tetrahedron - a solid with four faces - have Cartesian coordinates , where .

Give its volume in terms of .

Explanation:

A tetrahedron is a triangular pyramid and can be looked at as such.

Three of the vertices -  - are on the horizontal plane , and can be seen as the vertices of the triangular base. This triangle, as seen below, is isosceles:

Its base is 12 and its height is 15, so its area is

The fourth vertex is off this plane; its perpendicular (vertical) distance to the aforementioned face is the difference between the -coordinates, , so this is the height of the pyramid. The volume of the pyramid is

### Example Question #6 : Tetrahedrons

In three-dimensional space, the four vertices of a tetrahedron - a solid with four faces - have Cartesian coordinates .

Give its volume in terms of .

The correct answer is not among the other choices.

Explanation:

A tetrahedron is a triangular pyramid and can be looked at as such.

Three of the vertices -  - are on the horizontal plane , and can be seen as the vertices of the triangular base. This triangle, as seen below, is isosceles (drawing not to scale):

Its base is 20 and its height is 9, so its area is

The fourth vertex is off this plane; its perpendicular (vertical) distance to the aforementioned face is the difference between the -coordinates, , so this is the height of the pyramid. The volume of the pyramid is

### Example Question #7 : Tetrahedrons

In three-dimensional space, the four vertices of a tetrahedron - a solid with four faces - have Cartesian coordinates .

What is the volume of this tetrahedron?

Explanation:

The tetrahedron looks like this:

is the origin and  are the other three points, which are twelve units away from the origin, each on one of the three (mutually perpendicular) axes.

This is a triangular pyramid, so look at  as its base; the area  of the base is half the product of its legs, or

.

The volume of the tetrahedron, it being essentially a pyramid, is one third the product of its base and its height, the latter of which is 12. Therefore,

.

### Example Question #8 : Tetrahedrons

Above is the base of a triangular pyramid, which is equilateral. , and the pyramid has height 30. What is the volume of the pyramid?

Explanation:

Altitude  divides  into two 30-60-90 triangles.

By the 30-60-90 Theorem, , or

is the midpoint of , so

The area of the triangular base is half the product of its base and its height:

The volume of the pyramid is one third the product of this area and the height of the pyramid:

### Example Question #9 : Tetrahedrons

A regular tetrahedron has edges of length 4. What is its surface area?

Explanation:

A regular tetrahedron has four faces, each of which is an equilateral triangle. Therefore, its surface area, given sidelength , is

.

Substitute :

### Example Question #10 : Tetrahedrons

A regular tetrahedron comprises four faces, each of which is an equilateral triangle. Each edge has length 16. What is its surface area?