PSAT Math : Solid Geometry

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #12 : Prisms

A right rectangular prism has dimensions of 12.4 x 2.3 x 33. What is its surface area?

Possible Answers:

470.58

941.16

513.62

1882.32

1027.24

Correct answer:

1027.24

Explanation:

There are six faces to a right, rectangular prism. Based on our dimensions, we know that we must have a face that is 12.4 x 2.3, a face that is 2.3 x 33 and a face that is 33 x 12.4. To think this through, imagine that the front face is 12.4 x 2.3, the left side is 2.3 x 33, and the top is 33 x 12.4. Now, each of these sides has a matching side opposite (the left has the right, the top has the bottom, the front has the back).

Therefore, we know we have the following areas for the faces of our prism:

2 * 12.4 * 2.3 = 57.04

2 * 2.3 * 33 = 151.8

2 * 12.4 * 33 = 818.4

Add these to get the total surface area:

57.04 + 151.8 + 818.4 = 1027.24

Example Question #11 : Prisms

The dimensions of a right rectangular prism are such that the second dimension is twice the length of the first and the third is twice the length of the second. If the volume of the prism is 216 cubic units, what is its surface area?

Possible Answers:

126 square units

189 square units

215 square units

None of the other answers

252 square units

Correct answer:

252 square units

Explanation:

Based on our prompt, we can say that the prism has dimensions that can be represented as:

Dim1: x

Dim2: 2 * Dim1 = 2x

Dim3: 2 * Dim2 = 2 * 2x = 4x

More directly stated, therefore, our dimensions are: x, 2x, and 4x. Therefore, the volume is x * 2x * 4x = 216, which simplifies to 8x3 = 216 or x3 = 27. Solving for x, we find x = 3. Therefore, our dimensions are:

x = 3

2x = 6

4x = 12

Or: 3 x 6 x 12

Now, to find the surface area, we must consider that this means that our prism has sides of the following dimensions: 3 x 6, 6 x 12, and 3 x 12. Since each side has a "matching" side opposite it, we know that we have the following values for the areas of the faces:

2 * 3 * 6 = 36

2 * 6 * 12 = 144

2 * 3 * 12 = 72

The total surface area therefore equals: 36 + 144 + 72 = 252 square units.

Example Question #3 : How To Find The Surface Area Of A Prism

The area of a given object is 30,096 in2. What is the area of this object in ft2?

Possible Answers:

None of the other answers

209 ft2

2508 ft2

1254 ft2

1881 ft2

Correct answer:

209 ft2

Explanation:

Converting squared units is not difficult, though you have to be careful not to make a simple mistake. It is tempting to think you can merely divide the initial value (30,096) by 12, as though you were converting from inches to feet.

Begin by thinking this through as follows. In the case of a single dimension, we know that:

1 ft = 12 in   or   1 in = (1/12) ft

Now, think the case of a square with dimensions 1 ft x 1 ft. This square has the following dimensions in inches: 12 in x 12 in. The area is therefore 12 * 12 = 144 in2. This holds for all two-dimensional conversions. Therefore, the two dimensional conversion equation is:

1 ft2 = 144 in2   or   1 in2 = (1/144) ft2

Based on this, we can convert our value 30,096 in2 thus: 30,096/144 = 209 ft2.

Example Question #4 : How To Find The Surface Area Of A Prism

The area of a given object is 24 yd2. What is the area of this object in in2?

Possible Answers:

20,736 in2

None of the other answers

10,368 in2

31,104 in2

864 in2

Correct answer:

31,104 in2

Explanation:

Converting squared units is not difficult, though you have to be careful not to make a simple mistake. It is tempting to think you can merely multiply the initial value (24) by 36, as though you were converting from yards to inches.

Begin by thinking this through as follows. In the case of a single dimension, we know that:

1 yd = 36 in

Now, think the case of a square with dimensions 1 yd x 1 yd. This square has the following dimensions in inches: 36 in x 36 in. The area is therefore 36 * 36 = 1296 in2. This holds for all two-dimensional conversions. Therefore, the two dimensional conversion equation is:

1 yd2 = 1296 in2

Based on this, we can convert our value 24 yd2 thus: 24 * 1296 = 31,104 in2.

Example Question #13 : Prisms

Angie is painting a 2 foot cube for a play she is in. She needs 25\hspace{1 mm}mL of paint for every square foot she paints. How much paint does she need?

Possible Answers:

1.041\overline{6}\hspace{1 mm}mL

None of the available answers

600\hspace{1 mm}mL

It is impossible to convert between metric units and feet.

100\hspace{1 mm}mL

Correct answer:

600\hspace{1 mm}mL

Explanation:

First we must calculate the surface area of the cube. We know that there are six surfaces and each surface has the same area:

Area=6(2^2)=6\times 4=24\hspace{1 mm}feet^2

Now we will determine the amount of paint needed

24\hspace{1 mm}feet^2\times \frac{25\hspace{1 mm}mL}{1\hspace{1 mm}foot^2}=600\hspace{1 mm}mL

Example Question #84 : Solid Geometry

What is the surface area of an equilateral triangluar prism with edges of 6 in and a height of 12 in? 

Let  and .

Possible Answers:

Correct answer:

Explanation:

The surface area of the prism can be broken into three rectangular sides and two equilateral triangular bases.

The area of the sides is given by:  , so for all three sides we get .

The equilateral triangle is also an equiangular triangle by definition, so the base has congruent sides of 6 in and three angles of 60 degrees.  We use a special right traingle to figure out the height of the triangle: 30 - 60 - 90.  The height is the side opposite the 60 degree angle, so it becomes 3\sqrt{3} or 5.196. 

The area for a triangle is given by  and since we need two of them we get .

Therefore the total surface area is .

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

What is the total surface area of a box without a lid, if the dimensions of the base of the box are , and the box is  tall?

Possible Answers:

Correct answer:

Explanation:

The surface area of a rectangular prism is . Our prism, however, is missing its top, so it will be . This gives us .

Example Question #1 : Pyramids

The volume of a 6-foot-tall square pyramid is 8 cubic feet. How long are the sides of the base?

Possible Answers:

Correct answer:

Explanation:

Volume of a pyramid is

Thus:

Area of the base is .

Therefore, each side is .

Example Question #52 : Solid Geometry

A right pyramid with a square base has a height that is twice the length of one edge of the base.  If the height of the pyramid is 6 meters, find the volume of the pyramid.

Possible Answers:

30

18

6

24

12

Correct answer:

18

Explanation:

If the height, which is twice the length of the base edges, measures 6 meters, then each base edge must measure 3 meters.  

Since the base is a square, the area of the base is 3 x 3 = 9.  

Therefore the volume of the right pyramid is V = (1/3) x area of the base x height = 1/3(9)(6) = 18.

Example Question #1 : How To Find The Volume Of A Pyramid

What is the approximate volume of a square pyramid with one edge of the base measuring  and a height equal to the diagonal of its base?

Possible Answers:

Correct answer:

Explanation:

If one edge of the base is 2.7 inches, and the height of the pyramid is equal to the diagonal of the base, we can find the height using the edge. The height is going to be equal to . The area of the base is . Volume of a pyramid is equal to  

This leaves us with

 

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