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Question: Answered & Verified by Expert
2+5,1 are roots of the cubic equation given by
MathematicsQuadratic EquationAP EAMCETAP EAMCET 2021 (20 Aug Shift 2)
Options:
  • A x3+3x2-3x-1=0
  • B x3-3x2+3x-1=0
  • C x3-5x2+3x+1=0
  • D x3+5x2-3x+1=0
Solution:
1977 Upvotes Verified Answer
The correct answer is: x3-5x2+3x+1=0

We know that, in a cubic equation with rational coefficients has a irrational root α+β, where α and β are rational and β is not a perfect square, then it has also a conjugate root α-β.

Hence, 1, 2+5, 2-5are the roots of cubic equation.

We know that, if  α, β, γ are the roots of cubic equation, then its equation is x3-α+β+γx2+αβ+αγ+βγx-αβγ.

Let, α=1, β=2+5, γ=2-5

α+β+γ=1+2+5+2-5=5αβ+αγ+βγ==12+5+2+52-5+12-5=2+5+4-5+2-5=3

αβγ=12+52-5=-1.

Hence, required cubic equation is x3-5x2+3x+1=0.

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