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JEE Advanced 2016 Paper 1 · Mathematics · Quadratic Equations · medium

Let α,β,γ be the roots of x^3-2x^2+3x-5=0. Which of the following is/are correct?

One or more options correct

Let α,β,γ\alpha,\beta,\gamma be the roots of x3−2x2+3x−5=0x^3-2x^2+3x-5=0. Which of the following is/are correct?
  1. A. α2+β2+γ2=−2\alpha^2+\beta^2+\gamma^2=-2
  2. B. α3+β3+γ3=9\alpha^3+\beta^3+\gamma^3=9
  3. C. The equation has exactly one real root
  4. D. The cubic having roots α2,β2,γ2\alpha^2,\beta^2,\gamma^2 is x3+2x2−11x−25=0x^3+2x^2-11x-25=0
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Answer: A, C, D

From Vieta, e1=2e_1=2, e2=3e_2=3, e3=5e_3=5. Then ∑α2=e12−2e2=4−6=−2\sum\alpha^2=e_1^2-2e_2=4-6=-2, so (A) is true, and the negative value just reflects the complex roots. By Newton's identity, ∑α3=e1∑α2−e2e1+3e3=−4−6+15=5\sum\alpha^3=e_1\sum\alpha^2-e_2e_1+3e_3=-4-6+15=5, so (B) with 99 is false. The discriminant of the cubic is −367<0-367<0, so there is one real root and a complex pair; (C) is true. The new cubic has coefficients: sum of roots =−2=-2 giving x2x^2 coefficient +2+2; sum of products =e22−2e1e3=9−20=−11=e_2^2-2e_1e_3=9-20=-11; product =e32=25=e_3^2=25 giving constant −25-25. Hence x3+2x2−11x−25=0x^3+2x^2-11x-25=0, and (D) is true.
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