All Exams Test series for 1 year @ ₹349 only
Question

Directions: Consider the following for the next two (02) items that follow:

Let α and β be the roots of the equation x 2- (1 – 2a 2) x + (1 – 2a 2) = 0

Under what condition is \(\frac{1}{{{{\rm{\alpha }}^2}}} + \frac{1}{{{{\rm{\beta }}^2}}} < 1?\)

This question was previously asked in
NDA I 2016 GAT Previous Year Paper (17-Apr-2016)
The correct answer is \({{\rm{a}}^2} < \frac{1}{2}\)

Let the given quadratic equation be

\[x^2 - (1 - 2a^2)x + (1 - 2a^2) = 0\]

Let \(\alpha\) and \(\beta\) be the roots of this equation. For a general quadratic equation \(Ax^2 + Bx + C = 0\), Vieta's formulas give the sum and product of the roots:

  • Sum of roots: \(\alpha + \beta = -\frac{B}{A}\)
  • Product of roots: \(\alpha \beta = \frac{C}{A}\)

In our given equation, we have \(A=1\), \(B=-(1 - 2a^2)\), and \(C=(1 - 2a^2)\). Applying Vieta's formulas:

  • Sum of roots: \(\alpha + \beta = -\frac{-(1 - 2a^2)}{1} = 1 - 2a^2\)
  • Product of roots: \(\alpha \beta = \frac{1 - 2a^2}{1} = 1 - 2a^2\)

We are given the condition \(\frac{1}{{{\rm{\alpha }}^2}}} + \frac{1}{{{{\rm{\beta }}^2}}} < 1\). First, let's rewrite the left side of the inequality in terms of the sum and product of the roots:

\[\frac{1}{{{\rm{\alpha }}^2}}} + \frac{1}{{{{\rm{\beta }}^2}}} = \frac{{{\rm{\beta }}^2} + {{\rm{\alpha }}^2}}{{{{\rm{\alpha }}^2}{{\rm{\beta }}^2}}}\]

We know that \({{\rm{\alpha }}^2} + {{\rm{\beta }}^2} = {{({\rm{\alpha }} + {\rm{\beta }})}^2} - 2{\rm{\alpha }}{\rm{\beta }}\). Also, \({{\rm{\alpha }}^2}{{\rm{\beta }}^2} = {{({\rm{\alpha }}{\rm{\beta }})}^2}\).

So, the inequality becomes:

\[\frac{{{({\rm{\alpha }} + {\rm{\beta }})}^2} - 2{\rm{\alpha }}{\rm{\beta }}}{{{{({\rm{\alpha }}{\rm{\beta }})}^2}}} < 1\]

Now, substitute the expressions for \({\rm{\alpha }} + {\rm{\beta }}\) and \({\rm{\alpha }}{\rm{\beta }}\) in terms of \({\rm{a}}\):

\[\frac{{{(1 - 2{{\rm{a}}^2})}^2} - 2(1 - 2{{\rm{a}}^2})}{{{{(1 - 2{{\rm{a}}^2})}^2}}} < 1\]

Let \(y = 1 - 2{{\rm{a}}^2}\) for simplicity. The inequality is:

\[\frac{{{y}^2} - 2y}{{{y}^2}} < 1\]

Note that for the terms \(\frac{1}{{{\rm{\alpha }}^2}}}\) and \(\frac{1}{{{{\rm{\beta }}^2}}}\) to be defined, the roots \(\alpha\) and \(\beta\) must be non-zero. This implies that the product of roots \({\rm{\alpha }}{\rm{\beta }}\) must be non-zero. Thus, \(1 - 2{{\rm{a}}^2} \ne 0\), which means \(y \ne 0\).

Since \(y \ne 0\), we can simplify the inequality:

\[\frac{{{y}^2} - 2y}{{{y}^2}} - 1 < 0\]

\[\frac{{{y}^2} - 2y - {{y}^2}}{{{y}^2}} < 0\]

\[\frac{-2y}{{{y}^2}} < 0\]

\[\frac{-2}{y} < 0\]

For \(\frac{-2}{y} < 0\), since the numerator \(-2\) is negative, the denominator \(y\) must be positive. Therefore, we need \(y > 0\).

Substitute back \(y = 1 - 2{{\rm{a}}^2}\):

\[1 - 2{{\rm{a}}^2} > 0\]

\[1 > 2{{\rm{a}}^2}\]

\[{{\rm{a}}^2} < \frac{1}{2}\]

This condition \({\rm{a}}^2 < \frac{1}{2}\) also ensures that \(1 - 2{{\rm{a}}^2} > 0\), which means \(y \ne 0\), satisfying the requirement that the roots are non-zero.

We should also consider the nature of the roots. The discriminant of the quadratic equation is \(\Delta = B^2 - 4AC = {(-(1 - 2{{\rm{a}}^2}))^2} - 4(1)(1 - 2{{\rm{a}}^2}) = {(1 - 2{{\rm{a}}^2})}^2 - 4(1 - 2{{\rm{a}}^2})\). \(\Delta = (1 - 2{{\rm{a}}^2})[(1 - 2{{\rm{a}}^2}) - 4] = (1 - 2{{\rm{a}}^2})(-3 - 2{{\rm{a}}^2})\).

For \({\rm{a}}^2 < \frac{1}{2}\), we have \(1 - 2{{\rm{a}}^2} > 0\). Since \({\rm{a}}^2 \ge 0\), \(-3 - 2{{\rm{a}}^2}\) is always negative. Thus, \(\Delta = (1 - 2{{\rm{a}}^2})(-3 - 2{{\rm{a}}^2})\) will be negative when \({\rm{a}}^2 < \frac{1}{2}\). A negative discriminant means the roots are complex conjugates. If the roots are complex conjugates \(p \pm iq\) with \(q \ne 0\), their product is \(p^2 + q^2\), which is always positive. This matches \(1 - 2{{\rm{a}}^2} > 0\). Also, if the roots are complex, they cannot be zero, so \(\alpha \ne 0\) and \(\beta \ne 0\) is guaranteed if \(1 - 2a^2 \ne 0\). If \(1 - 2a^2 > 0\), then the roots are non-zero complex numbers, and our derivation holds.

If \({\rm{a}}^2 > \frac{1}{2}\), then \(1 - 2{{\rm{a}}^2} < 0\). In this case, \(-3 - 2{{\rm{a}}^2}\) is negative, so \(\Delta = (1 - 2{{\rm{a}}^2})(-3 - 2{{\rm{a}}^2})\) will be positive, meaning real and distinct roots. The product of roots \(1 - 2{{\rm{a}}^2}\) is negative. Our inequality \(\frac{-2}{y} < 0\) requires \(y > 0\), so \(1 - 2{{\rm{a}}^2} > 0\). This contradicts \(1 - 2{{\rm{a}}^2} < 0\) for \({\rm{a}}^2 > \frac{1}{2}\).

If \({\rm{a}}^2 = \frac{1}{2}\), then \(1 - 2{{\rm{a}}^2} = 0\). The equation becomes \(x^2 = 0\), with roots \(\alpha = \beta = 0\). The expression \(\frac{1}{{{\rm{\alpha }}^2}}} + \frac{1}{{{{\rm{\beta }}^2}}}\) is undefined.

Thus, the condition for the inequality \(\frac{1}{{{\rm{\alpha }}^2}}} + \frac{1}{{{{\rm{\beta }}^2}}} < 1\) is \({\rm{a}}^2 < \frac{1}{2}\).

Revision Table: Roots of Quadratic Equations and Vieta's Formulas

Concept Description Formula
Quadratic Equation A polynomial equation of the second degree. \(Ax^2 + Bx + C = 0\) (\(A \ne 0\))
Roots of Equation The values of \(x\) that satisfy the equation. \(\alpha, \beta\)
Sum of Roots Relation between coefficients and sum of roots. \(\alpha + \beta = -B/A\)
Product of Roots Relation between coefficients and product of roots. \(\alpha \beta = C/A\)
Discriminant Determines the nature of the roots. \(\Delta = B^2 - 4AC\)
Nature of Roots Based on Discriminant value. If \(\Delta > 0\), roots are real and distinct.
If \(\Delta = 0\), roots are real and equal.
If \(\Delta < 0\), roots are complex conjugates.

Additional Information: Manipulating Root Expressions

Expressions involving powers of roots like \(\alpha^2 + \beta^2\) or \(\frac{1}{\alpha^2} + \frac{1}{\beta^2}\) can often be simplified using symmetric polynomial identities and Vieta's formulas.

  • \({{\rm{\alpha }}^2} + {{\rm{\beta }}^2} = {{({\rm{\alpha }} + {\rm{\beta }})}^2} - 2{\rm{\alpha }}{\rm{\beta }}\)
  • \({{\rm{\alpha }}^3} + {{\rm{\beta }}^3} = {{({\rm{\alpha }} + {\rm{\beta }})}^3} - 3{\rm{\alpha }}{\rm{\beta }}({\rm{\alpha }} + {\rm{\beta }})\)
  • \(\frac{1}{{\rm{\alpha }}} + \frac{1}{{\rm{\beta }}} = \frac{{\rm{\beta }} + {\rm{\alpha }}}{{\rm{\alpha }}{\rm{\beta }}}\)
  • \(\frac{1}{{{\rm{\alpha }}^2}}} + \frac{1}{{{{\rm{\beta }}^2}}} = \frac{{{\rm{\beta }}^2} + {{\rm{\alpha }}^2}}{{{{({\rm{\alpha }}{\rm{\beta }})}^2}}} = \frac{{{({\rm{\alpha }} + {\rm{\beta }})}^2} - 2{\rm{\alpha }}{\rm{\beta }}}{{{{({\rm{\alpha }}{\rm{\beta }})}^2}}}\)

These identities are useful for solving problems involving roots without actually finding the values of the roots themselves, relying only on the coefficients of the quadratic equation.

The condition that \(\frac{1}{{{\rm{\alpha }}^2}}} + \frac{1}{{{{\rm{\beta }}^2}}}\) exists requires \(\alpha \ne 0\) and \(\beta \ne 0\). This implies the constant term of the quadratic equation must be non-zero, i.e., \(C \ne 0\). In this problem, \(C = 1 - 2a^2\), so we must have \(1 - 2a^2 \ne 0\), meaning \(a^2 \ne 1/2\).

The final answer is the condition derived from the inequality: \({\rm{a}}^2 < \frac{1}{2}\).

Was this answer helpful?

Similar Questions

  1. Which one of the following statements is correct?
  2. If p and q are the non-zero roots of the equation x 2+ px + q = 0, then how many possible values can q have?

  3. The quadratic equation 3x 2- (k 2+ 5k)x + 3k 2- 5k = 0 has real roots of equal magnitude and opposite sign. Which one of the following is correct?

  4. Under what condition does the above equation have real roots?

  5. If both the roots of the equation x 2– 2kx + k 2- 4 = 0 lie between – 3 and 5, then which one of the following is correct?

  6. If c > 0 and 4a + c < 2b, then ax 2– bx + c = 0 has a root in which one of the following intervals?

  7. The roots α and β of a quadratic equation, satisfy the relations α + β = α 2+ β 2and αβ = α 2β 2. What is the number of such quadratic equations?

  8. Consider the following statements :
    I. \(\sqrt{x} + x + 1 = 0\) has two irrational roots.
    II. \(5\sqrt{x} - x - 4 = 0\) has two rational roots.
    Which of the statements given above is/are correct ?
  9. Consider the following

    1. α + β + αβ > 0

    2. α 2β + β 2α > 0

    Which of the above is/are correct?
  10. Consider the following:

    1. β < - α

    2. β < |α|

    Which of the above is/are correct?

Important Questions from Quadratic Equations

  1. Find the zeros of the function \(f(x) = (x - 17)(x - 7)\)
  2. Find the roots of the following equation
    $x^2 + 12x - 36 = 0$
  3. Which of the following equations are quadratic equations?
    I) $x^2 + 4x + 23 = x + \frac{33}{28}$
    II) $\frac{2}{3}x^2 + 6x + 2 = x^2 + 8$
    III) $6x^2 + 4x + 18 = 6x^2 - 6x$
  4. Find the roots of the following equation:
    $4x^2 + 4x - 3 = 0$
  5. Which of the following is a quadratic equation?
Need Expert Advice?
Test Series
NDA img
Defence
NDA 2026 Mock Test Series (Latest Pattern)
526 Tests 1 Tests Free
1175 Attempts
4.7(160)
English, Hindi

Start Your Preparation with Prepp Mobile App

Download the app from Google Play & App Store
Download the app from Google Play & App Store
Prepp Mobile App