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JEE Advanced 2017 Paper 2 · Mathematics · Quadratic Equations · medium

The value of (α^2/(βγ)+(β^2/(γα)+(γ^2/(αβ) is

Single correct option

Let α,β,γ\alpha,\beta,\gamma be the roots of x3−4x2+5x−1=0x^3-4x^2+5x-1=0. The value of α2βγ+β2γα+γ2αβ\dfrac{\alpha^2}{\beta\gamma}+\dfrac{\beta^2}{\gamma\alpha}+\dfrac{\gamma^2}{\alpha\beta} is
  1. A. 44
  2. B. 77
  3. C. 1010
  4. D. 1919
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Answer: (B) 77

Since αβγ=1\alpha\beta\gamma=1 (product of roots of the monic cubic is −(−1)=1-(-1)=1), each term simplifies as α2βγ=α3αβγ=α3\dfrac{\alpha^2}{\beta\gamma}=\dfrac{\alpha^3}{\alpha\beta\gamma}=\alpha^3. So the required sum equals α3+β3+γ3\alpha^3+\beta^3+\gamma^3. With e1=∑α=4e_1=\sum\alpha=4, e2=∑αβ=5e_2=\sum\alpha\beta=5, e3=αβγ=1e_3=\alpha\beta\gamma=1, the identity ∑α3=e13−3e1e2+3e3\sum\alpha^3=e_1^3-3e_1e_2+3e_3 gives 64−60+3=764-60+3=7. Forgetting the 3e33e_3 term gives the trap value 44, while 1919 is the value 43−4⋅42+5⋅4−14^3-4\cdot4^2+5\cdot4-1 obtained by wrongly evaluating the cubic at x=e1x=e_1. A numerical check of the three roots confirms the sum is 77. Hence the answer is 77. This is a typical Advanced-style reduction: recognise the hidden symmetric function, convert it to a power sum, and then use Newton's identities rather than finding the roots.
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