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Complex Numbers question

2018 · Shift 0 · Q29
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Complex Numbers question

2018 · Shift 0 · Q29

JEE MainMathematicsComplex NumbersMCQ+4 / −1
If α,β∈C\alpha ,\beta \in Cα,β∈C are the distinct roots of the equation x2 - x + 1 = 0, then α101+β107{\alpha ^{101}} + {\beta ^{107}}α101+β107 is equal to :
  1. A
    2
  2. B
    -1
  3. C
    0
  4. D
    1
View written solutionFree

Correct answer: D

  1. We are given that α,β\alpha, \betaα,β are the distinct roots of x2−x+1=0.x^2-x+1=0.x2−x+1=0.

  2. Solve the quadratic: x=1±1−42=1±i32.x=\frac{1\pm \sqrt{1-4}}{2}=\frac{1\pm i\sqrt{3}}{2}.x=21±1−4​​=21±i3​​. So the roots are α=1+i32,β=1−i32.\alpha=\frac{1+i\sqrt{3}}{2},\qquad \beta=\frac{1-i\sqrt{3}}{2}.α=21+i3​​,β=21−i3​​. These are complex cube roots of unity other than 111.

  3. From x2−x+1=0,x^2-x+1=0,x2−x+1=0, for a root xxx, we get x2−x+1=0  ⟹  x2=x−1.x^2-x+1=0 \implies x^2=x-1.x2−x+1=0⟹x2=x−1. Multiplying by (x+1)(x+1)(x+1), x3+1=0  ⟹  x3=−1.x^3+1=0 \implies x^3=-1.x3+1=0⟹x3=−1. Hence x6=1.x^6=1.x6=1. Therefore, α6=1,β6=1.\alpha^6=1,\qquad \beta^6=1.α6=1,β6=1.

  4. Reduce the exponents modulo 666: 101≡5(mod6),107≡5(mod6).101 \equiv 5 \pmod{6}, \qquad 107 \equiv 5 \pmod{6}.101≡5(mod6),107≡5(mod6). So, α101=α5,β107=β5.\alpha^{101}=\alpha^5,\qquad \beta^{107}=\beta^5.α101=α5,β107=β5. Thus α101+β107=α5+β5.\alpha^{101}+\beta^{107}=\alpha^5+\beta^5.α101+β107=α5+β5.

  5. Since α6=1\alpha^6=1α6=1 and β6=1\beta^6=1β6=1, α5=α−1,β5=β−1.\alpha^5=\alpha^{-1},\qquad \beta^5=\beta^{-1}.α5=α−1,β5=β−1. Therefore, α5+β5=1α+1β=α+βαβ.\alpha^5+\beta^5=\frac{1}{\alpha}+\frac{1}{\beta}=\frac{\alpha+\beta}{\alpha\beta}.α5+β5=α1​+β1​=αβα+β​.

  6. Using Vieta's formulas for x2−x+1=0,x^2-x+1=0,x2−x+1=0, we have α+β=1,αβ=1.\alpha+\beta=1,\qquad \alpha\beta=1.α+β=1,αβ=1. Hence, α+βαβ=11=1.\frac{\alpha+\beta}{\alpha\beta}=\frac{1}{1}=1.αβα+β​=11​=1.

  7. Therefore, α101+β107=1.\alpha^{101}+\beta^{107}=1.α101+β107=1.

So the correct option is D.

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