Cube Me!

Algebra Level 2

If x + 1 x = 1 x+\frac1x=-1 , then find x 1999 + 1 x 1999 x^{1999}+\frac{1}{x^{1999}}


The answer is -1.

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3 solutions

Barack Clinton
Feb 15, 2015

x + 1 x = 1 x 2 + x + 1 = 0 x+\frac1x=-1\Rightarrow x^2+x+1=0

( x 1 ) ( x 2 + x + 1 ) = 0 (x-1)(x^2+x+1)=0

x 3 1 = 0 x 3 k 1 x^3-1=0\Rightarrow x^{3k}\equiv 1

x 1999 + 1 x 1999 = x ( x 3 ) 666 + 1 x ( x 3 ) 666 = x + 1 x = 1 x^{1999}+\frac{1}{x^{1999}}=x\cdot (x^3)^{666}+\frac{1}{x\cdot(x^3)^{666}}=x+\frac1x=-1

For the equation x+1/x=-1, Solutions are imaginary cubic roots of 1 = ω and ω²

also ω³=1, if substitue ω or ω² in the below equation, you will get -1.

x^1999 + 1/x^1999= -1 if x= ω , ω²

Gamal Sultan
Feb 18, 2015

The solutions of the equation x+1/x=-1 are ω and ω² (the complex cubic roots of

unity) where ω³=1

x^1999 + 1/x^1999 = x . (x^3)^666 + 1/x . (x^3)^666 = x+1/x=-1

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