ISEE Upper Level Quantitative : Cubes

Study concepts, example questions & explanations for ISEE Upper Level Quantitative

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Example Questions

Example Question #1 : Cubes

Which is the greater quantity?

(a) The volume of a cube with surface area  inches

(b) The volume of a cube with diagonal  inches

Possible Answers:

(a) is greater.

(a) and (b) are equal.

(b) is greater.

It is impossible to tell from the information given.

Correct answer:

(b) is greater.

Explanation:

The cube with the greater sidelength has the greater volume, so we need only calculate and compare sidelengths.

(a) \(\displaystyle A=6s^{2}\), so the sidelength of the first cube can be found as follows:

\(\displaystyle A=6s^{2}\)

\(\displaystyle 6s^{2}= 864\)

\(\displaystyle 6s^{2} \div 6= 864 \div 6\)

\(\displaystyle s^{2} = 144\)

\(\displaystyle s = \sqrt{144 }= 12\) inches

(b) \(\displaystyle d^{2} = s^{2}+ s^{2}+ s^{2} = 3 s^{2}\) by an extension of the Pythagorean Theorem, so the sidelength of the second cube can be found as follows:

\(\displaystyle 3 s^{2}= d^{2}\)

\(\displaystyle 3 s^{2}= 21^{2}= 441\)

\(\displaystyle 3 s^{2}\div 3= 441 \div 3\)

\(\displaystyle s^{2}= 147\)

\(\displaystyle s=\sqrt{ 147}\)

Since \(\displaystyle 147 > 144\)\(\displaystyle \sqrt{147 }> \sqrt{144}\). The second cube has the greater sidelength and, subsequently, the greater volume. This makes (b) greater.

Example Question #1 : Cubes

Cube 2 has twice the sidelength of Cube 1; Cube 3 has twice the sidelength of Cube 2; Cube 4 has twice the sidelength of Cube 3.

Which is the greater quantity?

(a) The mean of the volumes of Cube 1 and Cube 4

(b) The mean of the volumes of Cube 2 and Cube 3

Possible Answers:

(a) is greater.

It cannot be determined from the information given.

(a) and (b) are equal.

(b) is greater.

Correct answer:

(a) is greater.

Explanation:

The sidelengths of Cubes 1, 2, 3, and 4 can be given values \(\displaystyle s, 2s, 4s, 8s\), respectively.

Then the volumes of the cubes are as follows:

Cube 1: \(\displaystyle V= s^{3}\)

Cube 2: \(\displaystyle V= (2s)^{3} = 8s^{3}\)

Cube 3: \(\displaystyle V= (4s)^{3} = 64s^{3}\)

Cube 4: \(\displaystyle V= (8s)^{3} = 512s^{3}\)

In both answer choices ask for a mean, so we can determine which answer (mean) is greater simply by comparing the sums of volumes.

(a) The sum of the volumes of Cubes 1 and 4 is \(\displaystyle s^{3}+ 512^{3} = 513s^{3}\).

(b) The sum of the volumes of Cubes 2 and 3 is \(\displaystyle 8s^{3}+ 64^{3} = 72s^{3}\).

Regardless of \(\displaystyle s\), the sum of the volumes of Cubes 1 and 4 is greater, and therefore, so is their mean.

Example Question #1 : Cubes

What is the volume of a cube with side length \(\displaystyle 7.236\) \(\displaystyle in\)? Round your answer to the nearest hundredth.

Possible Answers:

\(\displaystyle 123.41\) \(\displaystyle in^3\)

\(\displaystyle 378.87\) \(\displaystyle in^3\)

\(\displaystyle 612.32\) \(\displaystyle in^3\)

\(\displaystyle 314.16\) \(\displaystyle in^3\)

\(\displaystyle 452.31\) \(\displaystyle in^3\)

Correct answer:

\(\displaystyle 378.87\) \(\displaystyle in^3\)

Explanation:

This question is relatively straightforward. The equation for the volume of a cube is:

\(\displaystyle V = s^3\)

(It is like doing the area of a square, then adding another dimension!)

Now, for our data, we merely need to "plug and chug:"

\(\displaystyle v=7.236^3 =378.874760256\)

Example Question #13 : Solid Geometry

What is the volume of a cube on which one face has a diagonal of \(\displaystyle 2\) \(\displaystyle in\)?

Possible Answers:

\(\displaystyle 12\) \(\displaystyle in^3\)

\(\displaystyle \frac{1}{2\sqrt{2} }\) \(\displaystyle in^3\)

\(\displaystyle 12\sqrt{2}\) \(\displaystyle in^3\)

\(\displaystyle 2\sqrt{2}\)  \(\displaystyle in^3\)

\(\displaystyle 2\) \(\displaystyle in^3\)

Correct answer:

\(\displaystyle 2\sqrt{2}\)  \(\displaystyle in^3\)

Explanation:

One of the faces of the cube could be drawn like this:

 

Squarediagonal-2

Notice that this makes a \(\displaystyle 45-45-90\) triangle.

This means that we can create a proportion for the sides. On the standard triangle, the non-hypotenuse sides are both \(\displaystyle 1\), and the hypotenuse is \(\displaystyle \sqrt{2}\).  This will allow us to make the proportion:

\(\displaystyle \frac{1}{\sqrt{2}} = \frac{x}{2}\)

Multiplying both sides by \(\displaystyle 2\), you get:

\(\displaystyle x=\frac{2}{\sqrt{2}}\)

Recall that the formula for the volume of a cube is:

\(\displaystyle V = s^3\)

Therefore, we can compute the volume using the side found above:

\(\displaystyle V = (\frac{2}{\sqrt{2}})^3=\frac{8}{(\sqrt{2})^3}=\frac{8}{2\sqrt{2}}=\frac{4}{\sqrt2}\)

Now, rationalize the denominator:

\(\displaystyle \frac{4}{\sqrt2} * \frac{\sqrt{2}}{\sqrt{2}} = \frac{4\sqrt{2}}{2}= 2\sqrt{2}\)

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

The volume of a cube is 343 cubic inches. Give its surface area.

Possible Answers:

\(\displaystyle 343 \; \textrm{in}^{2}\)

\(\displaystyle 294 \; \textrm{in}^{2}\)

\(\displaystyle 196 \; \textrm{in}^{2}\)

\(\displaystyle 392 \; \textrm{in}^{2}\)

\(\displaystyle 242\; \textrm{in}^{2}\)

Correct answer:

\(\displaystyle 294 \; \textrm{in}^{2}\)

Explanation:

The volume of a cube is defined by the formula

\(\displaystyle V=s^{3}\)

where \(\displaystyle s\) is the length of one side.

If \(\displaystyle V=343\), then 

\(\displaystyle s^{3} = 343\)

and 

\(\displaystyle s = \sqrt[3]{343} = 7\)

So one side measures 7 inches. 

The surface area of a cube is defined by the formula

\(\displaystyle A = 6s^{2}\) , so

\(\displaystyle A = 6 \cdot 7^{2} = 294\)

The surface area is 294 square inches.

Example Question #5 : Cubes

What is the surface area of a cube with side length \(\displaystyle 5.7\) \(\displaystyle in\)?

Possible Answers:

\(\displaystyle 194.94\) \(\displaystyle in^2\)

\(\displaystyle 32.49\) \(\displaystyle in^2\)

\(\displaystyle 185.193\) \(\displaystyle in^2\)

\(\displaystyle 91.18\) \(\displaystyle in^2\)

\(\displaystyle 173.47\) \(\displaystyle in^2\)

Correct answer:

\(\displaystyle 194.94\) \(\displaystyle in^2\)

Explanation:

Recall that the formula for the surface area of a cube is:

\(\displaystyle SA = 6 * s^2\), where \(\displaystyle s\) is the length of a side of the cube. This equation is easy to memorize because it is merely a multiplication of a single side (\(\displaystyle s^2\)) by \(\displaystyle 6\) because a cube has \(\displaystyle 6\) equal sides.

For our data, we know that \(\displaystyle s = 5.7\); therefore, our equation is:

\(\displaystyle SA = 6 * 5.7^2 = 6 * 32.49 = 194.94\)

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

What is the surface area of a cube with a volume \(\displaystyle 512\) \(\displaystyle in^3\)?

Possible Answers:

\(\displaystyle 384\) \(\displaystyle in^2\)

\(\displaystyle 193.44\) \(\displaystyle in^2\)

\(\displaystyle 192\) \(\displaystyle in^2\)

\(\displaystyle 321\) \(\displaystyle in^2\)

\(\displaystyle 85.33\) \(\displaystyle in^2\)

Correct answer:

\(\displaystyle 384\) \(\displaystyle in^2\)

Explanation:

To solve this, first calculate the side length based on the volume given. Recall that the equation for the volume of a cube is:

\(\displaystyle V = s^3\), where \(\displaystyle s\) is the side length.

For our data, this gives us:

\(\displaystyle s^3 = 512\)

Taking the cube-root of both sides, we get:

\(\displaystyle s = 8\)

Now, use the surface area formula to compute the total surface area:

\(\displaystyle SA = 6 * s^2\), where \(\displaystyle s\) is the length of a side of the cube. This equation is easy to memorize because it is merely a multiplication of a single side (\(\displaystyle s^2\)) by \(\displaystyle 6\) because a cube has \(\displaystyle 6\) equal sides.

For our data, this gives us:

\(\displaystyle SA = 6*8^2 = 6*64=384\)

Example Question #1 : Cubes

What is the surface area of a cube with a volume \(\displaystyle 274.625\) \(\displaystyle in^3\)?

Possible Answers:

\(\displaystyle 126.75\) \(\displaystyle in^2\)

\(\displaystyle 344.13\) \(\displaystyle in^2\)

\(\displaystyle 549.25\) \(\displaystyle in^2\)

\(\displaystyle 253.5\) \(\displaystyle in^2\)

\(\displaystyle 631.13\) \(\displaystyle in^2\)

Correct answer:

\(\displaystyle 253.5\) \(\displaystyle in^2\)

Explanation:

To solve this, first calculate the side length based on the volume given. Recall that the equation for the volume of a cube is:

\(\displaystyle V = s^3\), where \(\displaystyle s\) is the side length.

For our data, this gives us:

\(\displaystyle s^3 = 274.625\)

Taking the cube-root of both sides, we get:

\(\displaystyle s = 6.5\)

(You will need to use a calculator for this.  If your calculator gives you something like \(\displaystyle 6.599999999999\) . . . it is okay to round. This is just the nature of taking roots!).

Now, use the surface area formula to compute the total surface area:

\(\displaystyle SA = 6 * s^2\), where \(\displaystyle s\) is the length of a side of the cube. This equation is easy to memorize because it is merely a multiplication of a single side (\(\displaystyle s^2\)) by \(\displaystyle 6\) because a cube has \(\displaystyle 6\) equal sides.

For our data, this gives us:

\(\displaystyle SA = 6*6.5^2 = 6*42.25=253.5\)

Example Question #5 : How To Find The Surface Area Of A Cube

What is the surface area for a cube with a diagonal length of \(\displaystyle 3\sqrt{3}\) \(\displaystyle in\)?

Possible Answers:

\(\displaystyle 28\sqrt{3}\)

\(\displaystyle 15\sqrt{6}\)

\(\displaystyle 27\)

\(\displaystyle 3\sqrt{3}\)

\(\displaystyle 54\)

Correct answer:

\(\displaystyle 54\)

Explanation:

Now, this could look like a difficult problem; however, think of the equation for finding the length of the diagonal of a cube. It is like the Pythagorean Theorem, just adding an additional dimension:

\(\displaystyle D = \sqrt{3s^2}\)

(It is very easy, because the three lengths are all the same: \(\displaystyle s\)).

So, we know this, then:

\(\displaystyle 3\sqrt{3}=\sqrt{3s^2}\)

To solve, you can factor out an \(\displaystyle s\) from the root on the right side of the equation:

\(\displaystyle 3\sqrt{3}=s\sqrt{3}\)

Just by looking at this, you can tell that the answer is:

\(\displaystyle s=3\)

Now, use the surface area formula to compute the total surface area:

\(\displaystyle SA = 6 * s^2\), where \(\displaystyle s\) is the length of a side of the cube. This equation is easy to memorize because it is merely a multiplication of a single side (\(\displaystyle s^2\)) by \(\displaystyle 6\) because a cube has \(\displaystyle 6\) equal sides.

For our data, this is:

\(\displaystyle SA = 6 * 3^2 = 6*9=54\)

Example Question #6 : How To Find The Surface Area Of A Cube

What is the volume of a cube with a diagonal length of \(\displaystyle 7\sqrt{3}\) \(\displaystyle in\)?

Possible Answers:

\(\displaystyle 343\) \(\displaystyle in^3\)

\(\displaystyle 1029\sqrt{3}\) \(\displaystyle in^3\)

\(\displaystyle 14\sqrt{3}\) \(\displaystyle in^3\)

\(\displaystyle 343\sqrt{3}\) \(\displaystyle in^3\)

\(\displaystyle 171.5\) \(\displaystyle in^3\)

Correct answer:

\(\displaystyle 343\) \(\displaystyle in^3\)

Explanation:

Now, this could look like a difficult problem.  However, think of the equation for finding the length of the diagonal of a cube. It is like the Pythagorean Theorem, just adding an additional dimension:

\(\displaystyle D = \sqrt{3s^2}\)

(It is very easy, because the three lengths are all the same: \(\displaystyle s\)).

So, we know this, then:

\(\displaystyle 7\sqrt{3}=\sqrt{3s^2}\)

To solve, you can factor out an \(\displaystyle s\) from the root on the right side of the equation:

\(\displaystyle 7\sqrt{3}=s\sqrt{3}\)

Just by looking at this, you can tell that the answer is:

\(\displaystyle s=7\)

Now, use the equation for the volume of a cube:

\(\displaystyle V = s^3\)

(It is like doing the area of a square, then adding another dimension!).

For our data, it is:

\(\displaystyle V = 7^3 = 343\)

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