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Q8(iii):
Factorise each of the following: (iii) $27 – 125a^3 – 135a + 225a^2$

Solution :

Initial Setup & Given Polynomial

We are tasked with factorising the following algebraic expression:

$P(a) = 27 - 125a^3 - 135a + 225a^2$

Step 1: Identifying the Relevant Algebraic Identity

By analyzing the polynomial, we observe the presence of perfect cubes ($27$ and $125a^3$) and alternating signs. This structural pattern strongly indicates the application of the standard cubic identity for the difference of two terms [Per the Binomial Theorem for exponent 3]:

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

To utilize this identity, we must map the terms of our given polynomial to the terms of the expansion.

Step 2: Extracting the Base Terms (Cube Roots)

We isolate the perfect cube terms to determine our candidate values for $x$ and $y$.

  • First term (Constant): The term $27$ is a perfect cube.
    $27 = (3)^3 \implies x = 3$
  • Second term (Cubic): The term $125a^3$ is a perfect cube.
    $125a^3 = (5a)^3 \implies y = 5a$

Step 3: Verifying the Cross Terms

To rigorously confirm that the polynomial fits the identity $(x - y)^3$, we must evaluate the intermediate cross terms $-3x^2y$ and $+3xy^2$ using our candidate values $x = 3$ and $y = 5a$.

  • Evaluating $-3x^2y$:
    $-3(3)^2(5a) = -3(9)(5a) = -27(5a) = -135a$
    [This perfectly matches the third term of the given polynomial].
  • Evaluating $+3xy^2$:
    $+3(3)(5a)^2 = 9(25a^2) = 225a^2$
    [This perfectly matches the fourth term of the given polynomial].

Visual Mapping of the Algebraic Identity

The following diagram illustrates the exact one-to-one mapping between the given polynomial terms and the components of the cubic identity.

27 - 125a³ - 135a + 225a² - y³ - 3x²y + 3xy² (3)³ - (5a)³ - 3(3)²(5a) + 3(3)(5a)²

Step 4: Synthesizing the Factorised Form

Since all terms perfectly align with the expansion of $(x - y)^3$, we can rewrite the original polynomial in its expanded identity form:

$27 - 125a^3 - 135a + 225a^2 = (3)^3 - (5a)^3 - 3(3)^2(5a) + 3(3)(5a)^2$

Applying the identity $(x - y)^3$, where $x = 3$ and $y = 5a$, we condense the expression into a single perfect cube:

$= (3 - 5a)^3$

To express this as a product of its linear factors, we write the binomial three times.

Final Solution: The completely factorised form of the polynomial is $(3 - 5a)(3 - 5a)(3 - 5a)$.


More Questions from Class 9 Mathematics Polynomials EXERCISE 2.4


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