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## Homework Statement

Find x.

[tex]x^4+4x^3+3x^2-20x-20=0[/tex]

## Homework Equations

## The Attempt at a Solution

[tex]x^4+4x^3+3x^2-20x-20=0[/tex]/:[tex]x^2[/tex]

[tex]x^2+4x+3-20\frac{1}{x}-20\frac{1}{x^2}=0[/tex]

[tex](x^2-20\frac{1}{x^2})+4(x-5\frac{1}{x})+3=0[/tex]

[tex]x-5\frac{1}{x}=y[/tex]

[tex]x^2-10+25\frac{1}{x^2}=y^2[/tex]

[tex]x^2+25\frac{1}{x^2}=y^2+10[/tex]

???? I can't substitute for [tex]x^2-20\frac{1}{x^2}[/tex]