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Cube roots and perfect cubes

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5112538
A large cube is built completely from \(64\) identical small wooden cubes. What is the ratio of the edge length of the large cube to the edge length of one small cube? Explain your reasoning.

Hints

- The same number of small cubes must fit along each of the three dimensions. - What number cubed equals \(64\)? - Use a cube root to find the number of small cubes along one edge.

Solution

1. The same number of small cubes must fit along the length, width, and height of the large cube. 2. The number of cubes along each edge is \(\sqrt[3]{64}=4\), because \(4\cdot4\cdot4=64\). 3. Therefore, each edge of the large cube is \(4\) times the edge length of a small cube.

Answer

The ratio is \(4{:}1\). Each edge of the large cube is \(4\) times as long as an edge of a small cube.
5111838
A cube has a volume of \(60\,\text{dm}^3\). a) Between which two consecutive whole numbers, in decimeters, is the cube's edge length? b) Which of those two whole numbers is the better estimate of the edge length? Justify your answer with a calculation.

Hints

- Use the relationship between a cube's volume and its edge length. - Find two consecutive perfect cubes that enclose \(60\). - Test the midpoint of the two possible whole-number estimates.

Solution

1. The edge length is \(\sqrt[3]{60}\,\text{dm}\). 2. Since \(3^3=27\) and \(4^3=64\), \(27<60<64\). Therefore, the edge length is between \(3\,\text{dm}\) and \(4\,\text{dm}\). 3. The midpoint between \(3\) and \(4\) is \(3.5\). Since \(3.5^3=42.875<60\), \(\sqrt[3]{60}>3.5\). 4. Therefore, the edge length is closer to \(4\,\text{dm}\) than to \(3\,\text{dm}\).

Answer

a) The edge length is between \(3\,\text{dm}\) and \(4\,\text{dm}\). b) \(4\,\text{dm}\) is the better estimate because \(3.5^3=42.875<60\), so \(\sqrt[3]{60}>3.5\).
5112258
A sculptor makes two stone cubes. The smaller cube has a volume of \(8\,\text{dm}^3\), and the larger cube has a volume of \(216\,\text{dm}^3\). What is the ratio of the larger cube's edge length to the smaller cube's edge length? Explain your reasoning.

Hints

- Take the cube root of each volume. - Compare the two edge lengths. - You can also find the cube root of the volume ratio.

Solution

1. The smaller cube's edge length is \(\sqrt[3]{8}=2\,\text{dm}\). 2. The larger cube's edge length is \(\sqrt[3]{216}=6\,\text{dm}\). 3. The ratio is \(6\div2=3\), so the larger cube's edge is \(3\) times as long. 4. Equivalently, the volume ratio is \(216\div8=27=3^3\), so the edge-length ratio is \(\sqrt[3]{27}=3\).

Answer

The larger cube's edge length is \(3\) times the smaller cube's edge length, so the ratio is \(3{:}1\).
5112308
A cube has a volume of \(0.008\,\text{m}^3\). a) Find its edge length in centimeters. b) Find its surface area in square centimeters.

Hints

- Convert the volume to cubic centimeters. - Find the cube root of the volume. - A cube has six congruent square faces.

Solution

1. Convert the volume: \(0.008\,\text{m}^3=8000\,\text{cm}^3\). 2. The edge length is \(\sqrt[3]{8000}=20\,\text{cm}\). 3. The surface area is \(6s^2=6(20^2)=2400\,\text{cm}^2\).

Answer

a) The edge length is \(20\,\text{cm}\). b) The surface area is \(2400\,\text{cm}^2\).
5118288
A rectangular prism has a volume of \(1.44\,\text{dm}^3\). Its rectangular base measures \(12\,\text{cm}\times80\,\text{mm}\). a) Find the prism's height in centimeters. b) A cube has the same volume. Can its edge length be a whole number of centimeters? Justify your answer without calculating the cube root.

Hints

- Convert all measurements to centimeter-based units. - Use \(V=Bh\) for part a. - Compare the volume with nearby perfect cubes for part b.

Solution

1. Convert the measurements: \(1.44\,\text{dm}^3=1440\,\text{cm}^3\) and \(80\,\text{mm}=8\,\text{cm}\). 2. The base area is \(12\cdot8=96\,\text{cm}^2\), so the height is \(1440\div96=15\,\text{cm}\). 3. A cube with a whole-number edge has a perfect-cube volume. Since \(11^3=1331\) and \(12^3=1728\), and \(1331<1440<1728\), \(1440\) is not a perfect cube.

Answer

a) The prism's height is \(15\,\text{cm}\). b) No. The cube's edge cannot be a whole number of centimeters because \(1440\) lies between the consecutive perfect cubes \(11^3\) and \(12^3\).
5112508
Cube A has a volume of \(0.008\,\text{dm}^3\). Cube B has a surface area of \(54\,\text{cm}^2\). Which cube has the longer edge? Also find the difference between their volumes in cubic centimeters.

Hints

- Convert both measurements to centimeter-based units. - Use a cube root to find Cube A's edge. - Use \(SA=6s^2\) to find Cube B's edge.

Solution

1. Convert Cube A's volume: \(0.008\,\text{dm}^3=8\,\text{cm}^3\). 2. Its edge length is \(\sqrt[3]{8}=2\,\text{cm}\). 3. For Cube B, \(6s^2=54\), so \(s^2=9\) and \(s=3\,\text{cm}\). 4. Cube B has the longer edge. Its volume is \(3^3=27\,\text{cm}^3\). 5. The volume difference is \(27-8=19\,\text{cm}^3\).

Answer

Cube B has the longer edge: \(3\,\text{cm}\), compared with \(2\,\text{cm}\) for Cube A. The volume difference is \(19\,\text{cm}^3\).

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