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Question

The value of $\frac{(5.4)^3 - 0.064}{(5.4)^2 + 2.16 + 0.16}$ is:

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper PDF (31-Jan-2021) (Shift 1)
The correct answer is
5

Simplifying Algebraic Expressions

The question asks for the value of the expression: $ \frac{(5.4)^3 - 0.064}{(5.4)^2 + 2.16 + 0.16} $

Recognizing Algebraic Identities

This expression resembles the algebraic identity for the difference of cubes:

$ a^3 - b^3 = (a - b)(a^2 + ab + b^2) $

Let's identify the terms:

  • Let $a = 5.4$. Then $a^3 = (5.4)^3$.
  • We need to find $b$ such that $b^3 = 0.064$. Since $(0.4)^3 = 0.4 \times 0.4 \times 0.4 = 0.16 \times 0.4 = 0.064$, we have $b = 0.4$.

Verifying the Denominator

Now, let's check if the denominator matches the term $a^2 + ab + b^2$:

  • $a^2 = (5.4)^2$
  • $ab = (5.4)(0.4) = 2.16$
  • $b^2 = (0.4)^2 = 0.16$

The denominator is $(5.4)^2 + 2.16 + 0.16$, which perfectly matches $a^2 + ab + b^2$.

Applying the Identity and Simplifying

Substitute $a = 5.4$ and $b = 0.4$ into the expression and apply the identity:

$ \frac{a^3 - b^3}{a^2 + ab + b^2} = \frac{(a - b)(a^2 + ab + b^2)}{a^2 + ab + b^2} $

The term $(a^2 + ab + b^2)$ cancels out, leaving:

$ a - b $

Calculating the Final Value

Substitute the values of $a$ and $b$ back:

$ 5.4 - 0.4 = 5 $

Therefore, the value of the expression is 5.

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