Respuesta :

ahonyek
I assume that x6-9x3+8=0 is x^6-9x^3+8=0

use u=x^3 as substitution 

x^6=x^3*x^3=u*u=u^2 

So the original equation is now: 
u^2-9u+8=0 

Solve this for u than solve u=x^3 to get the x solutions. 

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