The Purplemath Forums

by **buddy** on Mon Mar 23, 2009 11:29 pm

Xto 4th power -3X to 3rd power - 40x squared = 0 How do I set this up into a quadratic equation? There are no examples close to this in my book. Do I sub. U forX to 3rd? If can have the set up or a formula for working this I can get it.

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by **Reckoner** on Mon Mar 23, 2009 11:57 pm

buddy wrote:Xto 4th power -3X to 3rd power - 40x squared = 0 How do I set this up into a quadratic equation? There are no examples close to this in my book. Do I sub. U forX to 3rd? If can have the set up or a formula for working this I can get it.

I take it that the equation given is $x^4-3x^3-40x^2=0?$ The first thing you should do is see if you can find any common factors in the terms on the left-hand side. What can you factor out of $x^4-3x^3-40x^2?$

Once you have the left side factored completely, set each factor equal to zero (separately) and solve for $x.$ Since this is a fourth-degree polynomial, you will have at most four solutions.

Good luck!

by **buddy** on Sat Mar 28, 2009 9:48 pm

do i do x2 out f everything? then wat?

by **stapel_eliz** on Sun Mar 29, 2009 12:25 am

Yes, factor the x² out.

Then factor the remaining quadratic.

. . . . . $x^2\left(x^2\, -\, 3x\, -\,40)$

Hint: What factors of 40 are three units apart? :wink:

by **buddy** on Sun Mar 29, 2009 7:07 pm

got it thnx anyway

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Xto 4th power -3X to 3rd power - 40x squared = 0

Xto 4th power -3X to 3rd power - 40x squared = 0

Re: Xto 4th power -3X to 3rd power - 40x squared = 0

Re: Xto 4th power -3X to 3rd power - 40x squared = 0

Re: Xto 4th power -3X to 3rd power - 40x squared = 0