An algebra problem by Akeel Howell

Algebra Level 4

If x + 1 x = 3 x+\dfrac{1}{x}=\sqrt{3} , then the solutions for x x are all primitive n th n^\text{th} roots of unity.

Find n n .


The answer is 12.

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1 solution

Akeel Howell
Feb 8, 2017

We are given that x + 1 x = 3 x+\dfrac{1}{x}=\sqrt{3} . Multiplying by x x gives x 2 3 x + 1 = 0 x^2-\sqrt{3}x+1=0 .

Applying the quadratic formula, we see that x = 3 ± i 2 x = { 3 2 + i 2 , 3 2 i 2 } x = e π i / 6 , e 11 π i / 6 x = \dfrac{\sqrt{3}\pm i}{2} \\ \implies x = \displaystyle{\left\{\dfrac{\sqrt{3}}{2}+\dfrac{i}{2}, \space \dfrac{\sqrt{3}}{2}-\dfrac{i}{2}\right\}} \\ \therefore x = \large{\space e^{\pi i/6}, \space e^{11\pi i/6}}

Since an n t h n^{th} root of unity is defined as e 2 k π i / n \large{e^{2k\pi i/n}} for k = 1 , 2 , 3 , . . . , n k = 1,2,3,...,n , these are primitive ( 2 × 6 ) t h = 1 2 t h {(2\times 6)}^{th} = 12^{th} roots of unity.

FYI Avoid others jump through the unnecessary hoops to answer the question. I've removed asking for 5 n 5n .

Calvin Lin Staff - 4 years, 4 months ago

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