Engineering
Mathematics
Quadratic Equation and Nature of Roots
Question

 Let p and q be real numbers such that p 0, p3  q and p3  – q. If α and β are nonzero complex numbers satisfying α + β = – p and α3 + β3 = q, then a quadratic equation having  αβ   and   βα as its roots is

(p3 – q)x2 – (5p3 – 2q)x + (p3 – q) = 0

(p3 + q)x2 – (p3 + 2q)x + (p3 + q) = 0

(p3 + q)x2 – (p3 – 2q)x + (p3 + q) = 0

(p3 – q)x2 – (5p3 + 2q)x + (p3 – q) = 0

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Solution

 Sum =  αβ+βα    or   Sum =  α2+β2αβ ;  Given  α + β = – p   and  α3 + β3 = q

  α2+β2+2αβ=p2                   ....(1)

 (α+β)(α2+β2αβ)=q

 α2+β2αβ=qp             .....(2)

(1) - (2) given  3αβ=p2+qp=p3+qpαβ=p3+q3p

required equation is  x2(α2+β2αβ)x+1=0

from (2), we get   α2+β2αβ=qpαβ+1=3pqp(p3+q)+1=p32qp3+q

equation  x2(p32q)p3+qx+1=0

(p3 + q)x2 – (p3 – 2q)x + (p3 + q) = 0

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