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Find all the roots of the equation : $$(x^2+1)^2 = x(3x^2+4x+3)$$How do we find the roots in polynomials of degree > 2 ??

Also, In odd degree polynomials i use descartes rule of signs to predict the number of real roots.Here, however, i it does not give me any clue about the number of real/imaginary roots. Is there a better method ?

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3 Answers 3

up vote 5 down vote accepted

On rearrangement we have $x^4-3x^3-2x^2-3x+1=0$

Like this, divide either sides by $x^2$ as $x\ne0$ to get $$x^2+\frac1{x^2}-3\left(x+\frac1x\right)-2=0$$

Or, $$\left(x+\frac1x\right)^2-2-3\left(x+\frac1x\right)-2=0$$

Put $x+\frac1x=u$

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How can we assume $x\ne0$ –  Simar Sep 1 '13 at 8:01
    
@Simar, put $x=0$ in the given eqaution –  lab bhattacharjee Sep 1 '13 at 8:02
    
k. This gives me $x^2 - 3x - 2- 3/x + 1/x^2$ . Further? –  Simar Sep 1 '13 at 8:03
    
@Simar, please find the edited answer –  lab bhattacharjee Sep 1 '13 at 8:04
    
Ah! Got that. What about the number of real roots part ?(In general for higher degree polynomials?) –  Simar Sep 1 '13 at 8:10
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Hint: $$(x^2+1)^2-3x(x^2+1)-4x^2=0$$ $$(x^2+1-4x)(x^2+1+x)=0$$

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Great. Please help me on the second part of the question. –  Simar Sep 1 '13 at 8:09
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Another solution:

Solve the equation $(x^2+1)^2 = 3x(x^2+1)+4x^2$ with $x^2+1$ as an unknown: $$ x^2+1=\frac{3x\pm \sqrt{9x^2+16x^2}}{2}=\frac{3x\pm 5x}{2} $$ etc.

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