PSAT Math : Cubes

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #21 : Cubes

A cube has a surface area of 10m2. If a cube's sides all double in length, what is the new surface area?

Possible Answers:

320m2

40m2

640m2

20m2

80m2

Correct answer:

40m2

Explanation:

The equation for surface area of the original cube is 6s2. If the sides all double in length the new equation is 6(2s)2 or 6 * 4s2. This makes the new surface area 4x that of the old. 4x10 = 40m2

Example Question #2 : How To Find The Surface Area Of A Cube

If a cube has an area of , then what is the surface area of this cube?

Possible Answers:

Correct answer:

Explanation:

The area of a cube is equal to the measure of one edge cubed. If we take the cube root of , we get , so the edges of this cube measure ; therefore, one face of the cube has an area of  square inches, because .

There are  sides to a cube, so

Example Question #1 : Solid Geometry

If a cube is 3” on all sides, what is the length of the diagonal of the cube?

Possible Answers:

3√2

9

3√3

27

4√3

Correct answer:

3√3

Explanation:

General formula for the diagonal of a cube if each side of the cube = s

Use Pythagorean Theorem to get the diagonal across the base:

s2 + s2 = h2

And again use Pythagorean Theorem to get cube’s diagonal, then solve for d:

h2 + s2 = d2

s2 + s2 + s2 = d2

3 * s2 = d2

d = √ (3 * s2) = s √3

So, if s = 3 then the answer is 3√3

Example Question #1 : Cubes

A cube is inscribed in a sphere of radius 1 such that all 8 vertices of the cube are on the surface of the sphere.  What is the length of the diagonal of the cube?

Possible Answers:

8

2

1

(3)

(2)

Correct answer:

2

Explanation:

Since the diagonal of the cube is a line segment that goes through the center of the cube (and also the circumscribed sphere), it is clear that the diagonal of the cube is also the diameter of the sphere.  Since the radius = 1, the diameter = 2.

Example Question #1 : How To Find The Diagonal Of A Cube

What is the length of the diagonal of a cube with volume of 512 in3?

Possible Answers:

2√(6) in

4√(3) in

8√(3) in

8 in

None of the other answers

Correct answer:

8√(3) in

Explanation:

The first thing necessary is to determine the dimensions of the cube.  This can be done using the volume formula for cubes: V = s3, where s is the length of the cube. For our data, this is:

s3 = 512, or (taking the cube root of both sides), s = 8.

The distance from corner to corner of the cube will be equal to the distance between (0,0,0) and (8,8,8).  The distance formula for three dimensions is very similar to that of 2 dimensions (and hence like the Pythagorean Theorem):

d√( (x1 – x2)2 + (y1 – y2)2  + (z1 – z2)2

Or for our simpler case:

d = √( (x)2 + (y)2  + (z)2) = √( (s)2 + (s)2  + (s)2) = √( (8)2 + (8)2  + (8)2) = √( 64 + 64 + 64) = √(64 * 3) = 8√(3)

Example Question #2 : Solid Geometry

What is the length of the diagonal of a cube with volume of 1728 in3?

Possible Answers:

12√(3) in

3√(3) in

12 in

18 in

6√(3) in

Correct answer:

12√(3) in

Explanation:

The first thing necessary is to determine the dimensions of the cube.  This can be done using the volume formula for cubes: V = s3, where s is the length of the cube. For our data, this is:

s3 = 1728, or (taking the cube root of both sides), s = 12.

The distance from corner to corner of the cube will be equal to the distance between (0,0,0) and (12,12,12).  The distance formula for three dimensions is very similar to that of 2 dimensions (and hence like the Pythagorean theorem):

d = √( (x1 – x2)2 + (y1 – y2)2  + (z1 – z2)2

Or, for our simpler case:

d = √( (x)2 + (y)2  + (z)2) = √( (s)2 + (s)2  + (s)2) = √( (12)2 + (12)2  + (12)2) = √( 144 + 144 + 144) = √(3 * 144) = 12√(3) = 12√(3)

Example Question #241 : Geometry

What is the length of the diagonal of a cube with surface area of 294 in2?

Possible Answers:

21√(2)

14

21

None of the other answers

7√(3)

Correct answer:

7√(3)

Explanation:

 The first thing necessary is to determine the dimensions of the cube. This can be done using the surface area formula for cubes: A = 6s2, where s is the length of the cube. For our data, this is:

6s2 = 294

s2 = 49

(taking the square root of both sides) s = 7

The distance from corner to corner of the cube will be equal to the distance between (0,0,0) and (7,7,7). The distance formula for three dimensions is very similar to that of 2 dimensions (and hence like the Pythagorean Theorem):

d = √((x1 – x2)2 + (y1 – y2)2  + (z1 – z2)2

Or for our simpler case:

d = √((x)2 + (y)2  + (z)2) = √( (s)2 + (s)2  + (s)2) = √( (7)2 + (7)2  + (7)2) = √( 49 + 49 + 49) =  √(49 * 3) = 7√(3)

Example Question #22 : Cubes

A rectangular prism has a volume of 144 and a surface area of 192. If the shortest edge is 3, what is the length of the longest diagonal through the prism?

Possible Answers:

Correct answer:

Explanation:

The volume of a rectangular prism is .

We are told that the shortest edge is 3.  Let us call this the height. 

We now have , or .

Now we replace variables by known values:

Now we have:

We have thus determined that the other two edges of the rectangular prism will be 4 and 12.  We now need to find the longest diagonal.  This is equal to:

If you do not remember how to find this directly, you can also do it in steps.  You first find the diagonal across one of the sides (in the plane), by using the Pythagorean Theorem.  For example, we choose the side with edges 3 and 4.  This diagonal will be:

We then use a plane with one side given by the diagonal we just found (length 5) and the other given by the distance of the 3rd edge (length 12). 

This diagonal is then .

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

The number of square units in the surface area of a cube is twice as large as the number of cubic units in its volume. What is the cube's volume, in cubic units?

Possible Answers:

216

36

27

108

9

Correct answer:

27

Explanation:

The number of square units in the surface area of a cube is given by the formula 6s2, where s is the length of the side of the cube in units. Moreover, the number of cubic units in a cube's volume is equal to s3

Since the number of square units in the surface area is twice as large as the cubic units of the volume, we can write the following equation to solve for s:

6s2 = 2s3

Subtract 6s2 from both sides.

2s3 – 6s2 = 0

Factor out 2s2 from both terms.

2s2(s – 3) = 0

We must set each factor equal to zero. 

2s2 = 0, only if s = 0; however, no cube has a side length of zero, so s can't be zero.

Set the other factor, s – 3, equal to zero.

s – 3 = 0

Add three to both sides.

s = 3

This means that the side length of the cube is 3 units. The volume, which we previously stated was equal to s3, must then be 33, or 27 cubic units.

The answer is 27. 

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

What is the surface area of a cube whose volume is 512 cubic feet?

Possible Answers:

Correct answer:

Explanation:

In order to find the surface area of a cube, we need to solve for the length of each side, .

Recall the formula for volume:

Plug in what we know and solve for :

Now plug this value into the surface area formula:

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