If a, b and c \; (a \gt 0, c \gt 0) are in GP, then consider the following in respect of the equation ax^{2}+bx+c=0: 1. The equation has imaginary roots. 2. The ratio of the roots of the equation is 1:\omega where \omega is a cube root of unity. 3. The product of roots of the equation is (\frac{b^{2}}{a^{2}}). Which of the statements given above are correct?

  1. A. 1 and 2 only
  2. B. 2 and 3 only
  3. C. 1 and 3 only
  4. D. 1, 2 and 3

Correct Answer: D. 1, 2 and 3

Explanation

Since a, b, c are in GP, b^2 = ac. The discriminant is \Delta = b^2 - 4ac = ac - 4ac = -3ac. Because a \gt 0 and c \gt 0, -3ac \lt 0, so the roots are imaginary (Statement 1 is true). The roots are x = \frac{-b \pm i\sqrt{3ac}}{2a}. Since \sqrt{ac} = b, x = \frac{b}{a}(\frac{-1 \pm i\sqrt{3}}{2}) = \frac{b}{a}\omega and \frac{b}{a}\omega^2. Their ratio is \omega : \omega^2 = 1 : \omega (Statement 2 is true). The product of the roots is \frac{c}{a} = \frac{ac}{a^2} = \frac{b^2}{a^2} (Statement 3 is true).

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