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Q10(ii):
Factorise each of the following: (ii) $64m^3 – 343n^3$ [Hint : See Question 9.]

Solution :

Given Expression & Initial Setup

We are tasked with factorising the following algebraic expression:

$64m^3 - 343n^3$

Upon initial inspection, the expression consists of two terms separated by a minus sign. The variables $m$ and $n$ are both raised to the third power, which strongly indicates that we must evaluate the numerical coefficients to determine if they are also perfect cubes.

Step 1: Identifying Perfect Cubes

To apply the relevant algebraic identity, we must rewrite both terms entirely as perfect cubes in the form of $x^3$ and $y^3$.

  • First Term ($64m^3$): The prime factorization of $64$ is $2^6$, which can be grouped as $(2^2)^3 = 4^3$. Therefore, the entire first term can be rewritten as:
    $64m^3 = (4m)^3$
  • Second Term ($343n^3$): The prime factorization of $343$ is $7 \times 7 \times 7 = 7^3$. Therefore, the entire second term can be rewritten as:
    $343n^3 = (7n)^3$

Substituting these back into the original expression yields:

$(4m)^3 - (7n)^3$

Step 2: Stating the Relevant Algebraic Identity

The expression is now explicitly in the form of a difference of two cubes. [Per the fundamental algebraic identities of polynomials], the difference of two cubes is factored using the following formula:

$x^3 - y^3 = (x - y)(x^2 + xy + y^2)$

Structural Mapping: Difference of Cubes x³ - y³ = (x - y)(x² + xy + y²) (4m)³ - (7n)³ x = 4m y = 7n

Step 3: Substitution and Expansion

By mapping our specific terms to the identity, we establish the following equivalencies:

  • $x = 4m$
  • $y = 7n$

We now substitute these values directly into the right-hand side of the identity, $(x - y)(x^2 + xy + y^2)$:

$= (4m - 7n) \left[ (4m)^2 + (4m)(7n) + (7n)^2 \right]$

Step 4: Final Simplification

To achieve the final factorised form, we must expand the terms within the square brackets by applying the exponent rules [specifically $(ab)^n = a^n b^n$] and performing basic multiplication:

  • Square the first term: $(4m)^2 = 4^2 \cdot m^2 = 16m^2$
  • Multiply the two terms: $(4m)(7n) = 4 \cdot 7 \cdot m \cdot n = 28mn$
  • Square the second term: $(7n)^2 = 7^2 \cdot n^2 = 49n^2$

Substituting these simplified components back into our factored expression yields:

$= (4m - 7n)(16m^2 + 28mn + 49n^2)$

Final Solution: $(4m - 7n)(16m^2 + 28mn + 49n^2)$


More Questions from Class 9 Mathematics Polynomials EXERCISE 2.4


CBSE Solutions for Class 9 Mathematics Polynomials


Chapters in CBSE - Class 9 Mathematics


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