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NDA 2024
Exam
NDA
Year
2024
Subject
Mathematics
Chapter
Quadratic Equation

NDA 2024 · Mathematics · Quadratic Equation

If a, b and c(a 0, c 0) are in GP, then consider the following in...

If a, b and c(a>0,c>0)c(a \gt 0, c \gt 0) are in GP, then consider the following in respect of the equation ax2+bx+c=0:a x^2+b x+c=0: 1. The equation has imaginary roots. 2. The ratio of the roots of the equation is 1:ω1: \omega where ω\omega is a cube root of unity. 3. The product of roots of the equation is (b2a2)\left(\frac{b^2}{a^2}\right). Which of the statements given above are correct?

Options

  1. A1 and 2 only
  2. B2 and 3 only
  3. C1 and 3 only
  4. D1,2 and 3

Correct answer

D. 1,2 and 3

Step-by-step solution

Given that a,b,ca, b, c are in GP ∴b2=ac\therefore \quad b^2=a c Now, ax2+bx+c=0a x^2+b x+c=0D=b2−4ac=ac−4ac=−3ac<0\begin{aligned} \mathrm{D} & =b^2-4 a c \\ & =a c-4 a c=-3 a c \lt 0 \end{aligned}1. $\therefore$ Roots are imaginary (correct) Let, $\quad a=2, b=4, c=8\begin{aligned} \therefore \quad x^2+2 x+4 & =0 \\ \Rightarrow \quad x & =\frac{-2 \pm \sqrt{4-16}}{2}=-1 \pm \sqrt{3} \end{aligned}$ 2. Ratio of roots $=\frac{-1+\sqrt{3}}{-1-\sqrt{3}}=\frac{\omega}{\omega^2}=\frac{1}{\omega}$ (correct) 3.\begin{aligned} \text{ Product of roots } & =\frac{(-1+\sqrt{3})(-1-\sqrt{3})}{2} \times 4 \\ & =\omega \cdot \omega^2 \cdot 4=4=\frac{b^2}{a^2} \end{aligned}$$ (correct) Hence, statements 1,2 and 3 are correct.
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