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Q8(iv):
Factorise each of the following: (iv) $64a^3 – 27b^3 – 144a^2b + 108ab^2$

Solution :

Step 1: Initial Setup and Polynomial Rearrangement

We are given the following polynomial expression of degree 3:

$P(a,b) = 64a^3 - 27b^3 - 144a^2b + 108ab^2$

To facilitate the identification of standard algebraic structures, we rearrange the terms in descending order of the powers of the first variable, $a$, and ascending order of the powers of the second variable, $b$.

$P(a,b) = 64a^3 - 144a^2b + 108ab^2 - 27b^3$

Step 2: Identification of the Relevant Algebraic Identity

The rearranged polynomial consists of four terms, with alternating signs ($+, -, +, -$), and the first and last terms appear to be perfect cubes. This structural signature strongly indicates the application of the standard binomial expansion identity for the cube of a difference [Per the Binomial Theorem for $(x-y)^n$ where $n=3$]:

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

Step 3: Extracting the Base Terms ($x$ and $y$)

We equate the first and last terms of our polynomial to the corresponding terms in the identity to find the base variables $x$ and $y$.

  • First Term ($x^3$):
    $x^3 = 64a^3$
    Taking the principal cube root of both sides:
    $x = \sqrt[3]{64a^3} = \sqrt[3]{4^3 \cdot a^3} = 4a$
  • Last Term ($y^3$):
    $y^3 = 27b^3$
    Taking the principal cube root of both sides:
    $y = \sqrt[3]{27b^3} = \sqrt[3]{3^3 \cdot b^3} = 3b$

Step 4: Verification of the Cross Terms

To rigorously prove that the polynomial is indeed a perfect cube, we must verify that the middle terms of the given expression exactly match the $-3x^2y$ and $+3xy^2$ terms of the identity when $x = 4a$ and $y = 3b$.

  • Verifying the second term ($-3x^2y$):
    $-3x^2y = -3(4a)^2(3b)$
    $= -3(16a^2)(3b)$
    $= -48a^2(3b) = -144a^2b$
    [This perfectly matches the second term of our rearranged polynomial.]
  • Verifying the third term ($+3xy^2$):
    $+3xy^2 = 3(4a)(3b)^2$
    $= 3(4a)(9b^2)$
    $= 12a(9b^2) = 108ab^2$
    [This perfectly matches the third term of our rearranged polynomial.]

Step 5: Structural Mapping and Visualization

Since all four terms map perfectly to the $(x - y)^3$ identity, we can rewrite the entire polynomial in its unexpanded structural form.

$64a^3 - 144a^2b + 108ab^2 - 27b^3 = (4a)^3 - 3(4a)^2(3b) + 3(4a)(3b)^2 - (3b)^3$

64a³ - 144a²b + 108ab² - 27b³ (4a)³ - 3(4a)²(3b) + 3(4a)(3b)² - (3b)³ (4a - 3b)³

Step 6: Final Factorization

By applying the identity $(x - y)^3$, we condense the expression into a single perfect cube:

$P(a,b) = (4a - 3b)^3$

To express this as a product of irreducible linear factors (which is the standard convention for complete factorization), we expand the exponent:

$P(a,b) = (4a - 3b)(4a - 3b)(4a - 3b)$

Final Solution: The completely factorised form of the polynomial $64a^3 - 27b^3 - 144a^2b + 108ab^2$ is $(4a - 3b)(4a - 3b)(4a - 3b)$.


More Questions from Class 9 Mathematics Polynomials EXERCISE 2.4


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