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Quadratic Equation and Inequalities question

2022 · 26 Jun · Shift 1 · Q33
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  5. /2022 · 26 Jun · Shift 1 · Q33

Quadratic Equation and Inequalities question

2022 · 26 Jun · Shift 1 · Q33

JEE MainMathematicsQuadratic Equation and InequalitiesNumerical+4 / −1
The sum of the cubes of all the roots of the equation x4−3x3−2x2+3x+1=0{x^4} - 3{x^3} - 2{x^2} + 3x + 1 = 0x4−3x3−2x2+3x+1=0 is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 36

  1. Let the roots of x4−3x3−2x2+3x+1=0x^4-3x^3-2x^2+3x+1=0x4−3x3−2x2+3x+1=0 be α,β,γ,δ\alpha,\beta,\gamma,\deltaα,β,γ,δ.

We need to find: α3+β3+γ3+δ3.\alpha^3+\beta^3+\gamma^3+\delta^3.α3+β3+γ3+δ3.

  1. Use Newton's sums / symmetric sums.

For the monic polynomial x4−3x3−2x2+3x+1=0,x^4-3x^3-2x^2+3x+1=0,x4−3x3−2x2+3x+1=0, by Vieta's formulas: α+β+γ+δ=3\alpha+\beta+\gamma+\delta=3α+β+γ+δ=3 αβ+αγ+αδ+βγ+βδ+γδ=−2\alpha\beta+\alpha\gamma+\alpha\delta+\beta\gamma+\beta\delta+\gamma\delta=-2αβ+αγ+αδ+βγ+βδ+γδ=−2 αβγ+αβδ+αγδ+βγδ=−3\alpha\beta\gamma+\alpha\beta\delta+\alpha\gamma\delta+\beta\gamma\delta=-3αβγ+αβδ+αγδ+βγδ=−3 αβγδ=1.\alpha\beta\gamma\delta=1.αβγδ=1.

Let p1=α+β+γ+δ,p_1=\alpha+\beta+\gamma+\delta,p1​=α+β+γ+δ, p2=α2+β2+γ2+δ2,p_2=\alpha^2+\beta^2+\gamma^2+\delta^2,p2​=α2+β2+γ2+δ2, p3=α3+β3+γ3+δ3.p_3=\alpha^3+\beta^3+\gamma^3+\delta^3.p3​=α3+β3+γ3+δ3.

We already have p1=3.p_1=3.p1​=3.

  1. First compute p2p_2p2​ using p2=(α+β+γ+δ)2−2(αβ+αγ+αδ+βγ+βδ+γδ).p_2=(\alpha+\beta+\gamma+\delta)^2-2(\alpha\beta+\alpha\gamma+\alpha\delta+\beta\gamma+\beta\delta+\gamma\delta).p2​=(α+β+γ+δ)2−2(αβ+αγ+αδ+βγ+βδ+γδ). So, p2=32−2(−2)=9+4=13.p_2=3^2-2(-2)=9+4=13.p2​=32−2(−2)=9+4=13.

  2. Now use Newton's sum for cubes: p3−e1p2+e2p1−3e3=0,p_3-e_1p_2+e_2p_1-3e_3=0,p3​−e1​p2​+e2​p1​−3e3​=0, where e1=3,e2=−2,e3=−3.e_1=3,\quad e_2=-2,\quad e_3=-3.e1​=3,e2​=−2,e3​=−3. Thus, p3−3(13)+(−2)(3)−3(−3)=0.p_3-3(13)+(-2)(3)-3(-3)=0.p3​−3(13)+(−2)(3)−3(−3)=0. Simplify: p3−39−6+9=0p_3-39-6+9=0p3​−39−6+9=0 p3−36=0p_3-36=0p3​−36=0 p3=36.p_3=36.p3​=36.

  3. Therefore, the sum of the cubes of all the roots is 36.\boxed{36}.36​.

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