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Quadratic Equation and Inequalities question

2010 · Shift 1 · Q33
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  4. /Quadratic Equation and Inequalities
  5. /2010 · Shift 1 · Q33

Quadratic Equation and Inequalities question

2010 · Shift 1 · Q33

JEE AdvancedMathematicsQuadratic Equation and InequalitiesMCQ+3 / −0.75
Let ppp and qqq be real numbers such that pe0, p3eqp e 0,\,{p^3} e qpe0,p3eq and p3e−q.{p^3} e - q.p3e−q. If p3e−q.{p^3} e - q.p3e−q. and  β\,\betaβ are nonzero complex numbers satisfying α +β=−p \alpha \, + \beta = - p\,α+β=−p and α3+β3=q,{\alpha ^3} + {\beta ^3} = q,α3+β3=q, then a quadratic equation having αβ{\alpha \over \beta }βα​ and βα{\beta \over \alpha }αβ​ as its roots is
  1. A
    (p3+q)x2−(p3+2q)x+(p3+q)=0\left( {{p^3} + q} \right){x^2} - \left( {{p^3} + 2q} \right)x + \left( {{p^3} + q} \right) = 0(p3+q)x2−(p3+2q)x+(p3+q)=0
  2. B
    (p3+q)x2−(p3−2q)x+(p3+q)=0\left( {{p^3} + q} \right){x^2} - \left( {{p^3} - 2q} \right)x + \left( {{p^3} + q} \right) = 0(p3+q)x2−(p3−2q)x+(p3+q)=0
  3. C
    (p3−q)x2−(5p3−2q)x+(p3−q)=0\left( {{p^3} - q} \right){x^2} - \left( {5{p^3} - 2q} \right)x + \left( {{p^3} - q} \right) = 0(p3−q)x2−(5p3−2q)x+(p3−q)=0
  4. D
    (p3−q)x2−(5p3+2q)x+(p3−q)=0\left( {{p^3} - q} \right){x^2} - \left( {5{p^3} + 2q} \right)x + \left( {{p^3} - q} \right) = 0(p3−q)x2−(5p3+2q)x+(p3−q)=0
View written solutionFree

Correct answer: B

  1. Given α+β=−p,α3+β3=q\alpha+\beta=-p, \qquad \alpha^3+\beta^3=qα+β=−p,α3+β3=q and we need the quadratic whose roots are αβ,βα.\frac{\alpha}{\beta},\quad \frac{\beta}{\alpha}. βα​,αβ​.

  2. Use the identity for sum of cubes α3+β3=(α+β)3−3αβ(α+β).\alpha^3+\beta^3=(\alpha+\beta)^3-3\alpha\beta(\alpha+\beta).α3+β3=(α+β)3−3αβ(α+β). Substituting the given values, q=(−p)3−3αβ(−p).q=(-p)^3-3\alpha\beta(-p).q=(−p)3−3αβ(−p). So, q=−p3+3pαβ.q=-p^3+3p\alpha\beta.q=−p3+3pαβ. Hence, 3pαβ=p3+q3p\alpha\beta=p^3+q3pαβ=p3+q and therefore αβ=p3+q3p.\alpha\beta=\frac{p^3+q}{3p}.αβ=3pp3+q​.

  3. Form the sum of the required roots Let r1=αβ,r2=βα.r_1=\frac{\alpha}{\beta},\qquad r_2=\frac{\beta}{\alpha}.r1​=βα​,r2​=αβ​. Then r1r2=1.r_1r_2=1.r1​r2​=1.

    Also, r1+r2=αβ+βα=α2+β2αβ.r_1+r_2=\frac{\alpha}{\beta}+\frac{\beta}{\alpha}=\frac{\alpha^2+\beta^2}{\alpha\beta}.r1​+r2​=βα​+αβ​=αβα2+β2​.

    Now, α2+β2=(α+β)2−2αβ=p2−2αβ.\alpha^2+\beta^2=(\alpha+\beta)^2-2\alpha\beta=p^2-2\alpha\beta.α2+β2=(α+β)2−2αβ=p2−2αβ. Thus, r1+r2=p2−2αβαβ=p2αβ−2.r_1+r_2=\frac{p^2-2\alpha\beta}{\alpha\beta}=\frac{p^2}{\alpha\beta}-2.r1​+r2​=αβp2−2αβ​=αβp2​−2.

  4. Substitute αβ\alpha\betaαβ Since αβ=p3+q3p,\alpha\beta=\frac{p^3+q}{3p},αβ=3pp3+q​, we get p2αβ=p2⋅3pp3+q=3p3p3+q.\frac{p^2}{\alpha\beta}=p^2\cdot \frac{3p}{p^3+q}=\frac{3p^3}{p^3+q}.αβp2​=p2⋅p3+q3p​=p3+q3p3​. Therefore, r_1+r_2=\frac{3p^3}{p^3+q}-2= rac{3p^3-2(p^3+q)}{p^3+q}= rac{p^3-2q}{p^3+q}.

  5. Write the quadratic For roots r1,r2r_1,r_2r1​,r2​, the quadratic is x2−(r1+r2)x+r1r2=0.x^2-(r_1+r_2)x+r_1r_2=0.x2−(r1​+r2​)x+r1​r2​=0. Hence, x2−p3−2qp3+qx+1=0.x^2-\frac{p^3-2q}{p^3+q}x+1=0.x2−p3+qp3−2q​x+1=0. Multiplying by p3+qp^3+qp3+q, (p3+q)x2−(p3−2q)x+(p3+q)=0.(p^3+q)x^2-(p^3-2q)x+(p^3+q)=0.(p3+q)x2−(p3−2q)x+(p3+q)=0.

  6. Match with the options This is exactly Option B.

  7. Verification with stored answer Stored correct answer: B

    Derived answer: B

    So the derived answer agrees with the stored answer.

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