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JEE Advanced 2008Paper 2
Exam
JEE Advanced
Year
2008
Paper
Paper 2
Subject
Mathematics
Chapter
Quadratic Equation

JEE Advanced 2008 · Paper 2 · Mathematics · Quadratic Equation

Let a, b, c, p, q be real numbers. Suppose, α, β are the roots of...

Let a,b,c,p,qa, b, c, p, q be real numbers. Suppose, α,β\alpha, \beta are the roots of the equation x2+2px+q=0x^2+2 p x+q=0 and α,1β\alpha, \frac{1}{\beta} are the roots of the equation ax2+2bx+c=0a x^2+2 b x+c=0, where β2∉{−1,0,1}\beta^2 \notin\{-1,0,1\}. Statement 1(p2−q)(b2−ac)≥01\left(p^2-q\right)\left(b^2-a c\right) \geq 0. Statement 2b≠pa2 b \neq p a or c≠qac \neq q a.

Options

  1. AStatement 1 is true, Statement 2 is true, Statement 2 is a correct explanation for Statement 1.
  2. BStatement 1 is true, Statement 2 is true, Statement 2 is not a correct explanation for Statement 1.
  3. CStatement 1 is true, Statement 2 is false.
  4. DStatement 1 is false, Statement 2 is true

Correct answer

B. Statement 1 is true, Statement 2 is true, Statement 2 is not a correct explanation for Statement 1.

Step-by-step solution

Given, α\alpha and β\beta are the roots of x2+2px+q=0x^2+2 p x+q=0. ∴α+β=−2p and αβ=q\begin{array}{rlrl}& \therefore \alpha+\beta& =-2 p \\ \text{ and } \alpha \beta& =q \end{array}\alphaandand\frac{1}{\beta}aretherootsofare the roots ofa x^2+2 b x+c=0andand\alpha+\frac{1}{\beta}=\frac{-2 b}{a}andand\frac{\alpha}{\beta}=\frac{c}{a} \text{ Now, } (p2−q)(b2−ac)=[(α+β−2)2−αβ][(α+1β2)2−αβ]a2=(α−β)216(α−1β)2\cdota2≥0\begin{aligned} \left(p^2-q\right)\left(b^2-a c\right) & =\left[\left(\frac{\alpha+\beta}{-2}\right)^2-\alpha \beta\right] \left[\left(\frac{\alpha+\frac{1}{\beta}}{2}\right)^2-\frac{\alpha}{\beta}\right]a^2 \\ & =\frac{(\alpha-\beta)^2}{16}\left(\alpha-\frac{1}{\beta}\right)^2 \cdota^2 \geq 0 \end{aligned} Statement 1 is true.  Again now, pa=−(α+β2)a=−a2(α+β) and b=−a2(α+1β)pa≠b⇒α+1β≠α+β⇒β≠1∵β2≠{−1,0,1}, correct.  Similarly, if c≠qx⇒aαβ\neqaαβ⇒α(β−1β)≠0\begin{array}{ll} \text{ Again now, }& p a=-\left(\frac{\alpha+\beta}{2}\right)a=-\frac{a}{2}(\alpha+\beta)\\ \text{ and }& b=-\frac{a}{2} \left(\alpha+\frac{1}{\beta}\right)\\ & p a ≠ b \Rightarrow \alpha+\frac{1}{\beta} \neq \alpha+\beta\\ \Rightarrow & \beta \neq1 \\ \because & \beta^2≠ \{-1,0,1\}, \text{ correct. }\\ \text{ Similarly, if }& c ≠ q x \\ \Rightarrow & a \frac{\alpha}{\beta} \neqa \alpha \beta \Rightarrow \alpha\left(\beta-\frac{1}{\beta}\right) \neq0 \end{array} ⇒α≠0 and  beta−1β≠0⇒β≠{−1,0,1}\begin{array}{ll} \Rightarrow& \alpha \neq0 \text{ and } \ beta-\frac{1}{\beta} \neq0 \\ \Rightarrow & \beta \neq\{-1,0,1\} \end{array} Statement 2 is true. Both Statement 1 and Statement 2 are true. But Statement 2 does not explain Statement 1.$
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