Posted by **Jaclyn** on Monday, September 12, 2011 at 5:27pm.

I know about Descartes Rule but I don't really get how to figure this out-Please check my answers-

I don't get this one find number of possible positive and negative real roots of f(x) = x^4-x^3+2x^2 + x-5

I think there are 3 sign changes so there are 3 positive but I get really confused on doing the negative real zeros would I rewrite for negative f(x) = -x^4 -(-x^3) -2x^2 -(x-5) so there would be 2negative real zeros

- Pre-Calc -
**Reiny**, Monday, September 12, 2011 at 6:03pm
f(x) = x^4 - x^3 + 2x^2 + x - 5

has 3 sign changes.

So there would be either 3 or 1 positive roots.

f(-x) = (-x)^4 - (-x)^3 + 2(-x)^2 + (-x) - 5

= x^4 + x^3 + 2x^2 - x - 5

I see only one sign change, so there is 1 negative roots.

So there would be either 1+ root, 1- root, and 2 complex

or

there would be 3+ roots, and 1- root

remember we could have at most 4 roots

Looks this youtube clip showing the procedure.

http://www.youtube.com/watch?v=5YAmwfT3Esc

running it through "wolfram" shows the roots

http://www.wolframalpha.com/input/?i=x%5E4-x%5E3%2B2x%5E2+%2B+x-5+%3D0

show 1 positive, 1 negative, and 2 complex roots

- Pre-Calc -
**Jaclyn**, Monday, September 12, 2011 at 7:37pm
Thank you-I viewed the youtube-that was really helpful and I did use Wolfram as you suggested. I even checked some of my other problems

## Answer This Question

## Related Questions

- Pre-Calc/Trig - Please check the first and help with the second-thank you Find ...
- Algebra 2/Trigonometry - How do I solve f(x)=x^3-6x^2+16x-96 using Descartes' ...
- college algebra--please help - what does Descartes rule of signs tell you about ...
- Algebra HELP! - Do not solve, show possibilities of Descartes's rule of signs ...
- Algebra - Do not solve, show possibilities of Descartes's rule of signs for ...
- Math - Is there any one who knows if I am right? Do not solve, show ...
- Pre Calculus - Using Descartes' Rule of Signs how can you determine the number ...
- College algebra - Use descartes rule of sign to determine the possible number ...
- Math Algebra - Use Descartes' Rule of Signs to determine the possible number of ...
- Pre-Calculus-check answers - Find the number of possible negative real zeros for...

More Related Questions