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 be the roots of x3−4x2+5x−1=0. The value of βγα2+γαβ2+αβγ2 is
A.4
B.7
C.10
D.19
Show answer and solution
Answer:(B) 7
Since αβγ=1 (product of roots of the monic cubic is −(−1)=1), each term simplifies as βγα2=αβγα3=α3. So the required sum equals α3+β3+γ3. With e1=∑α=4, e2=∑αβ=5, e3=αβγ=1, the identity ∑α3=e13−3e1e2+3e3 gives 64−60+3=7. Forgetting the 3e3 term gives the trap value 4, while 19 is the value 43−4⋅42+5⋅4−1 obtained by wrongly evaluating the cubic at x=e1. A numerical check of the three roots confirms the sum is 7. Hence the answer is 7. 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.