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Question

The roots of the equation $3x^{2} + 5x - a = 0$ are the reciprocals of each other. What is the value of $a$?

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
$-3$

Quadratic Equation Reciprocal Roots

For a general quadratic equation $Ax^2 + Bx + C = 0$, let the roots be $\alpha$ and $\beta$. The product of the roots is given by the formula $\alpha \beta = \frac{C}{A}$.

In this problem, the roots are stated to be reciprocals of each other. Let the roots be $\alpha$ and $\frac{1}{\alpha}$.

Applying the product of roots formula:

$ \alpha \times \frac{1}{\alpha} = \frac{C}{A} $

Simplifying the left side gives:

$ 1 = \frac{C}{A} $

This condition implies that $A = C$.

Finding 'a' in $3x^2 + 5x - a = 0$

Compare the given equation $3x^2 + 5x - a = 0$ with the standard form $Ax^2 + Bx + C = 0$.

  • The coefficient $A$ is $3$.
  • The coefficient $B$ is $5$.
  • The constant term $C$ is $-a$.

Determining the Value of 'a'

Using the condition $A = C$ derived from the reciprocal roots property:

$ 3 = -a $

To find the value of $a$, multiply both sides by $-1$:

$ a = -3 $

Thus, the value of $a$ is $-3$.

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Important Questions from Quadratic Equation

  1. If 2x 2+ 5x + 1 = 0, then one of the values of \(x - \frac{1}{{2x}}\)  is:

  2. If \(a-\frac{12}{a}=1\) , where a > 0, then the value of \(a^2+\frac{16}{a^2}\) is:

  3. If x 2 – 3x + 1 = 0, then the value of  \(\frac{(x^4+\frac{1}{x^2})}{(x^2+5x+1)}\)  is:

  4. If \(\sqrt{x}{}-{1\over\sqrt{x}}=\sqrt5\) \(x \ne 0\) , then what is the value of  \((x^4+{1\over{x^2}})\over(x^2+1) \)  ?

  5. If x 2\(\frac{1}{x^2}\)  = 18, x > 0, then find the value of x \(\frac{1}{x^3}\) .

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