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If \(\alpha\) and \(\beta\) are the roots of the equation \(\log_{10} [998+\sqrt{x^2-18x+76}] = 3\) then what is \((\alpha - \beta)^2\) equal to?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
36

To solve the problem of finding \((\alpha - \beta)^2\) where \(\alpha\) and \(\beta\) are roots of the equation \(\log_{10} [998+\sqrt{x^2-18x+76}] = 3\), let's follow these steps:

  1. Start with solving the logarithmic equation: \(\log_{10} [998+\sqrt{x^2-18x+76}] = 3\).
  2. Convert the logarithmic form to exponential form: \(998+\sqrt{x^2-18x+76} = 10^3\).
  3. Calculate the power of 10: \(10^3 = 1000\).
  4. Substitute back to find the equation: \(998+\sqrt{x^2-18x+76} = 1000\) leads to \(\sqrt{x^2-18x+76} = 2\).
  5. Square both sides to eliminate the square root: \(x^2 - 18x + 76 = 4\).
  6. Reorganize the equation into a standard quadratic form: \(x^2 - 18x + 72 = 0\).
  7. For a quadratic equation of the form \(ax^2 + bx + c = 0\), the roots satisfy \(\alpha + \beta = -\frac{b}{a}\) and \(\alpha\beta = \frac{c}{a}\). Here, \(a = 1, b = -18, c = 72\).
  8. Calculate \(\alpha + \beta = 18\) and \(\alpha \beta = 72\).
  9. Use the identity for the square of the difference of roots: \((\alpha - \beta)^2 = (\alpha + \beta)^2 - 4\alpha\beta\).
  10. Substitute the known values:
    • \((\alpha + \beta)^2 = 18^2 = 324\)
    • \(4\alpha\beta = 4 \times 72 = 288\)
  11. Calculate: \((\alpha - \beta)^2 = 324 - 288 = 36\).

The value of \((\alpha - \beta)^2\) is therefore 36.

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