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Cubes and Cube Roots - Properties of Cube Numbers

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

A cube of a number is obtained by multiplying the number by itself three times. Geometrically, if you have a solid cube with side length nn, its volume is represented by n3n^3. For example, a cube with side 33 units consists of 3×3×3=273 \times 3 \times 3 = 27 unit cubes.

A natural number is called a perfect cube if it is the cube of some natural number. In terms of prime factorization, a number is a perfect cube only if each prime factor appears in groups of three (triplets). For instance, 216=2×2×2×3×3×3216 = 2 \times 2 \times 2 \times 3 \times 3 \times 3, which can be grouped as 23×332^3 \times 3^3.

The units digit of a cube follows a specific pattern based on the units digit of the number: Numbers ending in 0,1,4,5,6,90, 1, 4, 5, 6, 9 have cubes ending in the same digits. However, numbers ending in 22 have cubes ending in 88 (and vice versa), and numbers ending in 33 have cubes ending in 77 (and vice versa).

The cube of an even number is always even, and the cube of an odd number is always odd. For example, 43=644^3 = 64 (even) and 53=1255^3 = 125 (odd).

Cubes can be expressed as the sum of consecutive odd numbers. 13=11^3 = 1, 23=3+52^3 = 3 + 5, 33=7+9+113^3 = 7 + 9 + 11, and so on. To find n3n^3, you sum nn consecutive odd numbers following the ones used for (n1)3(n-1)^3.

If a number ends with mm zeros, its cube ends with 3m3m zeros. For example, 1010 (1 zero) has a cube of 1,0001,000 (3 zeros), and 200200 (2 zeros) has a cube of 8,000,0008,000,000 (6 zeros). This visualizes how the volume expands cubically in all three dimensions.

Cube root is the inverse operation of finding a cube. If n3=mn^3 = m, then the cube root of mm, denoted as m3\sqrt[3]{m}, is nn. For example, since 53=1255^3 = 125, then 1253=5\sqrt[3]{125} = 5.

📐Formulae

n3=n×n×nn^3 = n \times n \times n

Volume of a cube=s3\text{Volume of a cube} = s^3

x3=x13\sqrt[3]{x} = x^{\frac{1}{3}}

(ab)3=a3×b3(ab)^3 = a^3 \times b^3

(ab)3=a3b3(\frac{a}{b})^3 = \frac{a^3}{b^3}

💡Examples

Problem 1:

Determine if 500500 is a perfect cube. If not, find the smallest natural number by which 500500 must be multiplied so that the product is a perfect cube.

Solution:

Step 1: Find the prime factorization of 500500. 500=2×250=2×2×125=2×2×5×5×5500 = 2 \times 250 = 2 \times 2 \times 125 = 2 \times 2 \times 5 \times 5 \times 5 Step 2: Group the factors into triplets. 500=(2×2)×(5×5×5)500 = (2 \times 2) \times (5 \times 5 \times 5) Step 3: Identify the missing factor. The prime factor 22 does not appear in a triplet (it only appears twice). Step 4: To make it a triplet, we need one more 22. Required number = 22. New product = 500×2=1000500 \times 2 = 1000, which is 10310^3.

Explanation:

To be a perfect cube, every prime factor must occur in a group of three. Since 22 occurred only twice, multiplying by 22 completes the triplet.

Problem 2:

Find the cube root of 80008000 using prime factorization.

Solution:

Step 1: Perform prime factorization of 80008000. 8000=2×4000=2×2×2000=2×2×2×10008000 = 2 \times 4000 = 2 \times 2 \times 2000 = 2 \times 2 \times 2 \times 1000 1000=2×2×2×5×5×51000 = 2 \times 2 \times 2 \times 5 \times 5 \times 5 So, 8000=(2×2×2)×(2×2×2)×(5×5×5)8000 = (2 \times 2 \times 2) \times (2 \times 2 \times 2) \times (5 \times 5 \times 5) Step 2: Take one factor from each triplet to find the cube root. 80003=2×2×5=20\sqrt[3]{8000} = 2 \times 2 \times 5 = 20.

Explanation:

The cube root is found by grouping prime factors into triplets and selecting one representative from each group to multiply together.