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Question

For the following two (02) items : 

Consider the equation $abx^2 + bcx + ca = cax^2 + abx + bc$

If the roots of the equation are equal, then which one of the following is correct?

This question was previously asked in
NDA 2 2025 GAT Question Paper (14-Sep-2025)
The correct answer is

\(\frac{1}{a} + \frac{1}{c} = \frac{2}{b}\) 

To solve this problem, we need to determine the condition for which the roots of the equation are equal. The given equation is:

\(abx^2 + bcx + ca = cax^2 + abx + bc\)

First, let's rearrange and simplify the given equation so that it is in the form of a standard quadratic equation:

  1. Subtract the right side from the left side:
  2. \(abx^2 + bcx + ca - (cax^2 + abx + bc) = 0\)

Rearranging terms, we get:

  1. \((ab - ca)x^2 + (bc - ab)x + (ca - bc) = 0\)

We recognize this as a quadratic equation of the form \(Ax^2 + Bx + C = 0\), where:

  • \(A = ab - ca\)
  • \(B = bc - ab\)
  • \(C = ca - bc\)

For roots of the quadratic equation to be equal, the discriminant must be zero. The discriminant for a quadratic equation \(Ax^2 + Bx + C = 0\) is given by:

\(\Delta = B^2 - 4AC\)

Set the discriminant equal to zero:

\((bc - ab)^2 - 4(ab - ca)(ca - bc) = 0\)

Expanding and simplifying the above expression is intricate. Alternatively, we explore the given options to see which satisfies the condition of equal roots straightforwardly:

\(ac = b^2\) may not directly relate to \(\Delta = 0\).

\(\frac{1}{a} + \frac{1}{c} = \frac{2}{b}\) implies a specific relationship often associated with symmetric properties that can simplify the equation, potentially leading to \(\Delta = 0\).

\(\frac{1}{a} + \frac{1}{c} = \frac{1}{2b}\) suggests another relational angle but does not directly aid the discriminant condition. 

\(a + c = 2b\) indicates symmetry but also doesn’t directly address the discriminant simplification.

Hence, the correct answer, based on symmetry and simplification in relation to \(\Delta = 0\), is:

\(\frac{1}{a} + \frac{1}{c} = \frac{2}{b}\)

Therefore, the correct answer is the second option.

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Similar Questions

  1. If the graph of a quadratic polynomial lies entirely above x-axis, then which one of the following is correct?

  2. If the roots of the equation x 2+ px + q = 0 are tan 19° and tan 26°, then which one of the following is correct?

  3. If x 2- px + 4 > 0 for all real values of x, then which one of the following is correct?

  4. How many real roots does the equation x 2+ 3|x| + 2 = 0 have?

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  6. In solving a problem that reduces to a quadratic equation, one student makes a mistake in the constant term and obtains 8 and 2 for roots. Another student makes a mistake only in the coefficient of first-degree term and finds -9 and -1 for roots.

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Important Questions from Quadratic Equations

  1. If the graph of a quadratic polynomial lies entirely above x-axis, then which one of the following is correct?

  2. If the roots of the equation x 2+ px + q = 0 are tan 19° and tan 26°, then which one of the following is correct?

  3. If x 2- px + 4 > 0 for all real values of x, then which one of the following is correct?

  4. If 2 + i is a root of the equation x 2- ax + 1 = 0, then the value of a is:

  5. If \(\tan \left( {\frac{α }{2}} \right)\)  and  \(\tan \left( {\frac{β }{2}} \right)\)  are the roots of the equation 8x 2- 26x + 15 = 0, then the value of cos (α + β) will be

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