Complete Solutions and Summary of Cubes and Cube Roots – NCERT Class 8 Mathematics Chapter 6
Detailed explanations, interesting number patterns, prime factorisation methods, examples, and exercises on cubes, cube roots, and Hardy-Ramanujan numbers from NCERT Class 8 Mathematics Chapter 6.
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Cubes and Cube Roots
Chapter 6: Mathematics — Complete Study Guide
Clear concepts, worked examples, summaries & exam-style questions
Chapter Overview
What you'll learn
Definition & Patterns
What are cubes and cube roots; patterns in cube endings and sums of odd numbers giving cubes.
Prime-factor method
How to test perfect cubes and find cube roots via grouping prime factors in triples.
Smallest multiplier/divisor
How to find the least number to multiply/divide to get a perfect cube (by adjusting prime exponents to multiples of 3).
Applications
Volume-side relationships of cubes and useful numeric identities (e.g., sums of consecutive odd numbers).
Quick context
This chapter introduces cubes (numbers of form \(n^3\)) and cube roots (the inverse operation). You will see examples, interesting numerical patterns (like Hardy–Ramanujan type facts), and techniques to decide if a number is a perfect cube and to compute cube roots using prime factorisation. Practical tasks include finding the least multiplier or divisor to make a number a cube and interpreting relationship between volume and side of cubes.
Comprehensive Chapter Summary
1. Introduction to cubes
Cube of a number \(n\) is \(n^3 = n \times n \times n\). Perfect cubes among small numbers: \(1, 8, 27, 64, 125, 216, 343, 512, 729, 1000,\dots\). The chapter highlights interesting numbers like 1729 — the smallest number expressible as sum of two cubes in two different ways (\(1729 = 1^3+12^3 = 9^3+10^3\)). :contentReference[oaicite:0]{index=0}
2. Recognising perfect cubes
Prime factor rule
A number is a perfect cube if, in its prime factorisation, every prime exponent is a multiple of 3. Example: \(216=2^3\cdot3^3\) so \(216=(2\cdot3)^3=6^3\). :contentReference[oaicite:1]{index=1}
Unit-digit patterns
The one's digit of cubes follows patterns (e.g., numbers ending in 2 have cubes ending in 8). Try examples like \(3331^3\), \(1024^3\). :contentReference[oaicite:2]{index=2}
3. Smallest multiplier / divisor to get a cube
To make a number \(N\) a perfect cube, adjust its prime exponents so each becomes a multiple of 3. If a prime \(p\) appears \(e\) times and \(e \bmod 3 = r\), you multiply by \(p^{(3-r)}\). Similarly, to divide to get a cube, divide out the primes whose exponents cannot be grouped in threes. Worked examples: 392 requires multiplication by 7 to become \(2744=14^3\); 1188 must be divided by 44 to yield 27.
4. Cube roots
Cube root is denoted \(\sqrt[3]{\cdot}\). Using prime factorisation, group primes in triples and multiply the base primes left after grouping. Examples: \(\sqrt[3]{3375}=15\) since \(3375=3^3\cdot5^3\). Other examples: \(\sqrt[3]{8000}=20,\ \sqrt[3]{13824}=24.\)
5. Patterns & identities
Some numeric patterns: sums of consecutive odd numbers give cubes (e.g., \(1=1^3,\;3+5=2^3,\;7+9+11=3^3\)). Differences of cubes follow a factor pattern: \(a^3-b^3=(a-b)(a^2+ab+b^2)\). Explore ending-digit behaviour and growth comparisons like \(m^2 < m^3\) for suitable integers. :contentReference[oaicite:5]{index=5}
6. What we practiced
- Identify perfect cubes, compute cube roots by prime factor method.
- Find least numbers to multiply/divide to obtain perfect cubes.
- Understand patterns: Hardy–Ramanujan number, unit-digit patterns, sums of odd numbers producing cubes.
Key Concepts & Formulas
Cube
\(n^3 = n \times n \times n\). Example: \(5^3 = 125\).
Cube root
\(\sqrt[3]{x}\) is the number whose cube is \(x\). E.g. \(\sqrt[3]{27}=3\).
Prime-factor method
Write \(N=\prod p_i^{e_i}\). If every \(e_i\) is multiple of 3, \(N\) is a perfect cube. \(\sqrt[3]{N}=\prod p_i^{e_i/3}\).
Make a cube
To make \(N\) a cube by multiplication, for each prime with \(e_i \bmod 3 = r\), multiply by \(p_i^{3-r}\).
Selected formulas (for book-like display)
Basic algebraic identities related to cubes (rendered via MathJax):
\(\displaystyle (a+b)^3 = a^3 + 3a^2b + 3ab^2 + b^3\)
\(\displaystyle a^3 - b^3 = (a-b)\bigl(a^2 + ab + b^2\bigr)\)
\(\displaystyle a^3 + b^3 = (a+b)\bigl(a^2 - ab + b^2\bigr)\)
Questions and Answers — Chapter 6
Short Questions (brief answers)
Q1. What is a perfect cube? Give two examples.
Q2. Is 500 a perfect cube?
Q3. Find \(\sqrt[3]{8000}\).
Q4. What is the cube of 12?
Q5. Give the cube root of 3375.
Q6. Is 243 a perfect cube?
Q7. State the prime-factor test for perfect cubes.
Q8. What is 20³?
Q9. What is the cube root of 13824?
Q10. If a number ends with digit 8, can it be a cube? (Give example or counter)
Q11. Is 27000 a perfect cube?
Q12. What is special about 1729?
Q13. How many cubes are there between 1 and 1000?
Q14. Is 10648 a perfect cube? If yes, what is its cube root?
Q15. What is the cube of 5?
Medium Questions — (Answer in ~3 marks)
Q1. Using prime factorisation find the cube root of 46656.
Q2. Find the smallest number by which 392 must be multiplied to get a perfect cube.
Q3. By which smallest number should 53240 be divided to obtain a perfect cube?
Q4. Find cube root of 110592 by prime factorisation (hint: used in exercises).
Q5. Find the smallest number by which 68600 must be multiplied to get a perfect cube.
Q6. Check whether 1000 and 9000 are perfect cubes.
Q7. Find \(\sqrt[3]{175616}\) by prime factorisation.
Q8. Express 63 as sum of consecutive odd numbers using the cube-sum pattern.
Q9. Decide true/false: "Cube of any odd number is even."
Q10. Find the cube root of 46656 using prime factorisation (exercise item).
Q11. If \(N = 2^4\cdot3^2\cdot5\), what smallest multiplier makes \(N\) a perfect cube?
Q12. Why is 243 not a cube though it is a high power of 3?
Q13. Find the cube root of 91125 from the exercise list.
Q14. Show that 343000 is a perfect cube and find its cube root.
Q15. For \(n\in\mathbb{N}\), check whether \(n^2 < n^3\) always holds. Explain.
Long Questions — (Detailed answers)
Q1. Explain in detail how to use prime factorisation to test whether a number is a perfect cube and find the smallest multiplier to make it a cube. Illustrate with 68600.
Step 1: Prime-factorise: \(68600 = 2^3\cdot5^2\cdot7^3\). Step 2: Check exponents: for a perfect cube each exponent must be multiple of 3. Here exponents are 3 (OK), 2 (not OK), 3 (OK). Step 3: For exponent 2, need \(3-2=1\) extra factor of 5. So multiply by \(5\) to get \(68600\times5=343000\). After multiplication prime exponents become \(2^3\cdot5^3\cdot7^3\), all multiples of 3. Hence \(343000=(2\cdot5\cdot7)^3=70^3\). So the smallest multiplier is 5 and cube root of resulting cube is 70. :contentReference[oaicite:31]{index=31}
Q2. Work out fully: find the smallest number by which 1188 must be divided so that quotient is a perfect cube and state the cube root of the quotient.
Factorise: \(1188=2^2\cdot3^3\cdot11\). Exponents: 2 (for 2), 3 (for 3), 1 (for 11). To make exponents multiples of 3 by division, remove the primes with exponents not divisible by 3 — remove \(2^2\) and \(11^1\) i.e. divide by \(2^2\cdot11 = 44\). Then quotient = \(1188/44 = 27 = 3^3\). Cube root = 3. So smallest divisor = 44. :contentReference[oaicite:32]{index=32}
Q3. Prove the formula \(a^3-b^3=(a-b)(a^2+ab+b^2)\) and give an example with numbers.
Expand RHS: \((a-b)(a^2+ab+b^2)=a^3+a^2b+ab^2 - a^2b - ab^2 - b^3 = a^3 - b^3\). Example: let \(a=5, b=2\): LHS \(=125-8=117\). RHS \((5-2)(25+10+4)=3\times39=117\). Identity holds.
Q4. Using the cube-sum pattern (sum of odd numbers), show how 125 can be obtained as a sum of consecutive odd numbers.
125 = \(5^3\). The pattern says sum of first \(n\) odd numbers equals \(n^2\), and certain consecutive-odd sums produce cubes: \(21+23+25+27+29=125\). Indeed, these five consecutive odd numbers sum to 125, exhibiting the pattern that the sum of \(n\) consecutive odd numbers (appropriately chosen) can give \(n^3\) for \(n=5\). (Chapter activities). :contentReference[oaicite:33]{index=33}
Q5. Demonstrate with full working how \(\sqrt[3]{74088}=42\) using prime factorisation.
Prime factorise \(74088 = 2^3\cdot3^3\cdot7^3\). Group each prime exponent into triples: \((2^3)(3^3)(7^3) = (2\cdot3\cdot7)^3\). Therefore \(\sqrt[3]{74088} = 2\cdot3\cdot7 = 42.\) :contentReference[oaicite:34]{index=34}
Q6. A cuboid has dimensions \(15\times30\times15\) cm. How many such cuboids are needed to make a perfect cube? Explain.
Volume of each cuboid \(=15\times30\times15=6750 = 2\times3^3\times5^3\) (prime factorisation gives one 2 and three 3s and three 5s). To make the combined volume a perfect cube, the exponent of 2 must be a multiple of 3. Currently exponent of 2 is 1; multiplying by 4 (i.e., using 4 such cuboids) provides \(2^3\). Thus 4 cuboids are required to make a cube. (See worked example). :contentReference[oaicite:35]{index=35}
Q7. Show with steps how to find \(\sqrt[3]{13824}\).
\(13824 = 2^9\cdot3^3\) (since \(13824 = 2^9\times3^3\)). Grouping gives \((2^3\cdot3)^3 = (8\cdot3)^3 = 24^3\). So cube root = 24. :contentReference[oaicite:36]{index=36}
Q8. From the chapter: list ten cubes from \(1^3\) to \(10^3\) and explain why only ten cubes lie below 1000.
1³=1, 2³=8, 3³=27, 4³=64, 5³=125, 6³=216, 7³=343, 8³=512, 9³=729, 10³=1000. Below 1000, only cubes of numbers 1–9 are strictly less than 1000; 10³ equals 1000. So there are 10 cubes from 1 to 1000 inclusive (1³ through 10³). :contentReference[oaicite:37]{index=37}
Q9. How do unit-digit patterns help to quickly reject non-cubes? Illustrate with 243 and 500.
Study of unit digits of cubes shows allowed endings (e.g. 0,1,8,7,4,5,6,3,2,9 based on base digit). But prime exponent check is faster: 243 = \(3^5\) (not cube), 500 = \(2^2\cdot5^3\) (2's exponent not multiple of 3) → both not cubes. Unit digits alone are heuristic; prime-factor test is conclusive. :contentReference[oaicite:38]{index=38}
Q10. Explain the concept of Hardy–Ramanujan numbers and give one example besides 1729.
Hardy–Ramanujan numbers are integers expressible as sum of two positive cubes in more than one way. 1729 is the smallest. Another example is 4104 which equals \(2^3+16^3=9^3+15^3\). These are interesting because they show different representations as sums of cubes. :contentReference[oaicite:39]{index=39}
Q11. Given \(N=2^5\cdot3^4\cdot7\), find the smallest number to multiply with \(N\) to get a perfect cube and find cube root of the result.
Exponents modulo 3: 5→2 so need 1 more 2; 4→1 so need 2 more 3s; 1→1 so need 2 more 7s. Multiply by \(2^1\cdot3^2\cdot7^2 = 2\cdot9\cdot49=882\). New number is cube of \(2^{(5+1)/3}3^{(4+2)/3}7^{(1+2)/3} = 2^2\cdot3^2\cdot7^1 = 4\cdot9\cdot7 = 252\). So cube root = 252. (Method illustrated).
Q12. Using examples from the chapter, explain how to find the least number to divide \(53240\) to get a perfect cube.
Prime factor \(53240 = 2^3\cdot11^3\cdot5^1\). Exponent of 5 is 1; dividing by 5 removes the lone 5, leaving all exponents multiples of 3. Thus divide by 5 → quotient \(10648=22^3\). So smallest divisor = 5. :contentReference[oaicite:40]{index=40}
Q13. Evaluate and explain: Does any perfect cube end with two zeros? (Reference tasks in the chapter.)
If a number ends with two zeros, it's divisible by 100 = \(2^2\cdot5^2\). For it to be a cube, exponents of 2 and 5 in prime factorisation must be multiples of 3. Since 2 and 5 each contribute at least exponent 2, we need extra factors to make them multiples of 3 (e.g., exponent 3 is possible if multiplied suitably). So while some numbers with two ending zeros can be cubes (e.g., 1000 ends with three zeros and is \(10^3\)), a cube ending with exactly two zeros is not possible because exponent-of-5 and 2 would not both be multiples of 3 with exactly two trailing zeros. (Chapter has related true/false items). :contentReference[oaicite:41]{index=41}
Q14. Prove that if each prime factor of a number appears three times, the number is a perfect cube. Give an example.
If \(N=\prod p_i^{3k_i}\) then \(N=\big(\prod p_i^{k_i}\big)^3\), hence a perfect cube. Example: \(216=2^3\cdot3^3=(2\cdot3)^3=6^3\). (Seen in chapter examples). :contentReference[oaicite:42]{index=42}
Q15. Discuss patterns observed in cubes of numbers ending with digits 1–9; illustrate how the one's digit of cube depends on unit digit of original number.
Compute single-digit cubes: \(0^3=0,1^3=1,2^3=8,3^3=27 (units 7),4^3=64 (units 4),5^3=125 (units 5),6^3=216 (units 6),7^3=343 (units 3),8^3=512 (units 2),9^3=729 (units 9)\). The unit digit of the cube is determined only by unit digit of the original number. This helps quick checks (chapter 'Try these' tasks). :contentReference[oaicite:43]{index=43}
Source (chapter used to prepare the above questions & answers):
Interactive Knowledge Quiz
Test your understanding of Cubes & Cube Roots
Quick Revision Notes
Key facts
- Cube \(= n^3\)
- Cube root \(\sqrt[3]{x}\)
- Prime-exponent multiples of 3 ⇒ perfect cube
Common cubes
- \(2^3=8\), \(3^3=27\), \(4^3=64\), \(5^3=125\)
- \(10^3=1000\), \(20^3=8000\), \(30^3=27000\)
How to make a cube
- Adjust prime exponents to multiples of 3
- Multiply by missing prime powers
Exam tips
- Use prime factorisation for exact cube roots
- Unit-digit rule for quick elimination
- Memorise small cubes table
Practice sequence
- Prime-factor small numbers and group exponents in 3s.
- Try the smallest-multiplier problems from the exercises.
- Use the identities for algebraic manipulations of cubes.
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