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EEJ MAIN Mathematics QUESTION #6039
Question 1

If $\alpha,\beta\in\mathbb{C}$ are distinct roots of $x^2-x+1=0$, find $\alpha^{101}+\beta^{107}$.

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Correct Answer Explanation

The roots of $x^2-x+1=0$ are $x=\frac{1\pm\sqrt{-3}}{2}=-\omega,-\omega^2$ where $\omega=e^{2\pi i/3}$.

Let $\alpha=-\omega, \beta=-\omega^2$. Then:

$\alpha^{101}=(-\omega)^{101}=-\omega^{101}=-\omega^{101\mod3}=-\omega^2$ (since $101=33\times3+2$)

$\beta^{107}=(-\omega^2)^{107}=-\omega^{214}=-\omega^{214\mod3}=-\omega^1$ (since $214=71\times3+1$)

$\alpha^{101}+\beta^{107}=-\omega^2-\omega=-(w+\omega^2)=-(-1)=\mathbf{1}$