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Q4(v):
Expand each of the following, using suitable identities: (v) $(–2x + 5y – 3z)^2$

Solution :

Initial Setup & Theoretical Foundation

We are tasked with expanding the following algebraic expression:

$(-2x + 5y - 3z)^2$

[Per the fundamental algebraic identity for the square of a trinomial], the expansion of any three-term polynomial squared is governed by the following formula, derived from the distributive property of multiplication over addition:

$(a + b + c)^2 = a^2 + b^2 + c^2 + 2ab + 2bc + 2ca$

Geometric Proof of (a + b + c)² a b c a b c a² ab ac ab b² bc ac bc c² Total Area = a² + b² + c² + 2ab + 2bc + 2ac

Step 1: Variable Mapping

To apply the identity correctly, we must map the components of our specific expression to the standard variables $a$, $b$, and $c$. It is critical to include the negative signs as part of the terms, as the standard identity assumes addition between all terms.

  • Let $a = -2x$
  • Let $b = 5y$
  • Let $c = -3z$

Step 2: Substitution into the Identity

Substituting the mapped variables into the expansion formula yields:

$(-2x + 5y - 3z)^2 = (-2x)^2 + (5y)^2 + (-3z)^2 + 2(-2x)(5y) + 2(5y)(-3z) + 2(-3z)(-2x)$

Step 3: Expansion and Simplification of Individual Terms

We now systematically evaluate the squares and the cross-product terms [adhering strictly to the rules of exponentiation and sign multiplication].

1. Evaluating the Squared Terms:

  • $a^2 = (-2x)^2 = (-2)^2 \cdot x^2 = 4x^2$
  • $b^2 = (5y)^2 = 5^2 \cdot y^2 = 25y^2$
  • $c^2 = (-3z)^2 = (-3)^2 \cdot z^2 = 9z^2$

[Note: The square of any real number, whether positive or negative, is always positive.]

2. Evaluating the Cross-Product Terms:

  • $2ab = 2 \cdot (-2x) \cdot (5y) = 2 \cdot (-10xy) = -20xy$
  • $2bc = 2 \cdot (5y) \cdot (-3z) = 2 \cdot (-15yz) = -30yz$
  • $2ca = 2 \cdot (-3z) \cdot (-2x) = 2 \cdot (6zx) = 12zx$

Step 4: Final Assembly of the Polynomial

Combining all the simplified terms from Step 3 into a single polynomial expression:

$4x^2 + 25y^2 + 9z^2 + (-20xy) + (-30yz) + 12zx$

Resolving the adjacent signs (positive and negative) simplifies the expression to its standard form:

$4x^2 + 25y^2 + 9z^2 - 20xy - 30yz + 12zx$

Final Solution: The expanded form of $(-2x + 5y - 3z)^2$ is $4x^2 + 25y^2 + 9z^2 - 20xy - 30yz + 12zx$.


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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