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Quadratic Equation and Inequalities question

2010 · Shift 0 · Q46
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  5. /2010 · Shift 0 · Q46

Quadratic Equation and Inequalities question

2010 · Shift 0 · Q46

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
If α\alphaα and β\betaβ are the roots of the equation x2−x+1=0,{x^2} - x + 1 = 0,x2−x+1=0, then α2009+β2009={\alpha ^{2009}} + {\beta ^{2009}} =α2009+β2009=
  1. A
     −1\, - 1−1
  2. B
     1\, 11
  3. C
     2\, 22
  4. D
     −2\, - 2−2
View written solutionFree

Correct answer: B

  1. Given quadratic equation

    x2−x+1=0x^2-x+1=0x2−x+1=0

    Let its roots be α\alphaα and β\betaβ.

  2. Find the roots

    Using the quadratic formula:

    x=1±1−42=1±i32x=\frac{1\pm\sqrt{1-4}}{2}=\frac{1\pm i\sqrt{3}}{2}x=21±1−4​​=21±i3​​

    So,

    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​​

  3. Write roots in polar form

    Observe that

    1+i32=cos⁡π3+isin⁡π3\frac{1+i\sqrt{3}}{2}=\cos\frac{\pi}{3}+i\sin\frac{\pi}{3}21+i3​​=cos3π​+isin3π​ 1−i32=cos⁡π3−isin⁡π3=cos⁡π3+isin⁡(−π3)\frac{1-i\sqrt{3}}{2}=\cos\frac{\pi}{3}-i\sin\frac{\pi}{3}=\cos\frac{\pi}{3}+i\sin\left(-\frac{\pi}{3}\right)21−i3​​=cos3π​−isin3π​=cos3π​+isin(−3π​)

    Hence,

    α=eiπ/3,β=e−iπ/3\alpha=e^{i\pi/3},\qquad \beta=e^{-i\pi/3}α=eiπ/3,β=e−iπ/3

  4. Raise to the power 200920092009

    α2009+β2009=ei⋅2009π/3+e−i⋅2009π/3\alpha^{2009}+\beta^{2009}=e^{i\cdot 2009\pi/3}+e^{-i\cdot 2009\pi/3}α2009+β2009=ei⋅2009π/3+e−i⋅2009π/3

    Using

    eiθ+e−iθ=2cos⁡θ,e^{i\theta}+e^{-i\theta}=2\cos\theta,eiθ+e−iθ=2cosθ,

    we get

    α2009+β2009=2cos⁡(2009π3)\alpha^{2009}+\beta^{2009}=2\cos\left(\frac{2009\pi}{3}\right)α2009+β2009=2cos(32009π​)

  5. Reduce the angle modulo 2π2\pi2π

    Since 2009≡5(mod6)2009\equiv 5\pmod{6}2009≡5(mod6),

    2009π3≡5π3(mod2π)\frac{2009\pi}{3}\equiv \frac{5\pi}{3} \pmod{2\pi}32009π​≡35π​(mod2π)

    Therefore,

    2cos⁡(2009π3)=2cos⁡(5π3)2\cos\left(\frac{2009\pi}{3}\right)=2\cos\left(\frac{5\pi}{3}\right)2cos(32009π​)=2cos(35π​)

    Now,

    cos⁡(5π3)=12\cos\left(\frac{5\pi}{3}\right)=\frac{1}{2}cos(35π​)=21​

    So,

    α2009+β2009=2⋅12=1\alpha^{2009}+\beta^{2009}=2\cdot \frac{1}{2}=1α2009+β2009=2⋅21​=1

  6. Check options

    The correct option is:

    B: 1\boxed{\text{B: }1}B: 1​

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