Roots of negative unity

Algebra Level 4

x 2 n + x 2 n 1 + x 2 n 2 + + x 2 + x + 1 \large x^{2n} + x^{2n-1} + x^{2n-2} + \dots + x^2 + x+ 1

In the polynomial above, n n is a positive integer. The roots of this polynomial can be expressed as ( 1 ) m α n + β + m (-1)^{\frac{m}{\alpha n + \beta}+m} where m m is an integer that is never equal to k ( α n + β ) k(\alpha n + \beta) where k k is odd, α \alpha and β \beta are positive integers where β < α < 8 \beta < \alpha < 8 .

Find α + β α β \cfrac{\alpha + \beta}{\alpha \beta}


The answer is 1.5.

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1 solution

Roots of the equation are the complex ( 2 n + 1 ) t h (2n+1)'^{th} roots of unity. Since we are interested in the denominator only, we see that α = 2 , β = 1 α=2,β=1

Hence α + β α β = 3 2 = 1.5 \dfrac {α+β}{αβ}=\dfrac 32=\boxed {1.5} .

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