How Do I Solve Polynomial Equations with Real and Complex Roots?

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Discussion Overview

The discussion revolves around solving a cubic polynomial equation, specifically finding the values of t for which the polynomial s(t) equals -30. Participants explore methods for factoring the polynomial, identifying roots, and applying various techniques for polynomial equations.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant presents the polynomial equation s(t) = 1/2t^3 - 5t^2 + 3t + 6 and seeks help in finding t when s(t) = -30.
  • Another participant suggests using polynomial division to factor the polynomial, indicating that t=4 is a known root.
  • A different participant reiterates the polynomial division approach and mentions using the quadratic formula to find the second root, t=8.196.
  • One participant expresses confusion about how to identify t=4 as a root and seeks clarification on the factoring process.
  • Another participant references Fermat's techniques for estimating polynomial roots and suggests examining integer factors of the constant term to find potential roots.
  • A participant mentions that setting the equation equal to -30 leads to a cubic equation that can be solved more easily.
  • One participant introduces Descartes' rule of signs to analyze the nature of the roots and suggests a method for identifying possible rational roots through testing factors of the leading coefficient and constant term.

Areas of Agreement / Disagreement

Participants generally agree on the methods of polynomial division and the use of integer factors to identify roots. However, there is no consensus on the best approach to factor the polynomial or identify roots, as different methods are proposed and explored.

Contextual Notes

Some participants express uncertainty about the steps involved in polynomial division and the reasoning behind identifying specific roots. There are also mentions of various techniques without a clear resolution on their effectiveness or applicability to the problem at hand.

Who May Find This Useful

This discussion may be useful for students learning about polynomial equations, those seeking strategies for factoring polynomials, and individuals interested in methods for finding roots of cubic equations.

crookesm
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Given:

[tex]s(t) = 1/2t^3-5t^2+3t+6[/tex]

I'm trying to find all values of t where s(t) = -30

My first thought is to solve for 0 hence:

[tex]1/2t^3-5t^2+3t+36=0[/tex]

I know the answers are t=4 and t=8.196 but I can't get to it...I'm assuming I need to factor this down but I'm can't see it. Any help/hints would be most appreciated as I've been banging my head against a brick wall for some time now...
 
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You know that 4 is a root of the polynomial in your last equation.
Use polynomial division to compete the factoring.
 
As above, but divide (x-4) into 1/2t^3-5t^2+3t+36

Then when you get the second polynomial use the quadratic solution to get t=8.196
 
Thanks for the hints - reading up on polynomial division (which I wasn't familiar with) I have found that the factors are:

[tex](t-4)(1/2t^2-3t-9)[/tex]

However, (and I realize I'm getting slightly off topic here) how would I even arrive at (t-4) being one of the original factors. Using GCF I can easily see that:

[tex](t)(1/2t^2-5t+3)+36 = 0[/tex]

But the jump to t-4 has got me stumped!

Any pointers to threads/websites on factoring polynomials/finding roots of polynomials would be most appreciated.
 
Last edited:
I believe Fermat developed various techniques in order to make good, reasoned guesses for the roots of polynomials.

However, when meeting a polynomial in a textbook that you don't immediately recognize the roots of, remember that the constant term is the product of the roots.

Therefore, one way of arriving at 4 as a root is to examine the integer factors of 36, and see which of these (if any!) might be a root.
 
putting the equation equal to -30 u can make a cubic equation which will be then easy to solve for
 
Use descarte's rule of signs to find the nature of the roots (positive, negative, or complex).

After that take X minus all factors (positive and negative) of the coefficient of the highest degree of your variable over the factors of your constant.

3X^3 + 5X^2 - 2X + 8 =0

so your possible (real) factors are going to be (3/8), (3/4), (3/2), 3, (1/8), (1/4), (1/2), 1 and their inverses.

Well, now you're left with 14 possible roots, you can a) try them all algorithmically, or b) apply some heuristics. Graph the polynomial (preferably on a calculator) and look at which of the roots seem to be true (where the funtion intersects the X-Axis)--After that, try each of your roots until you find the root you're looking for.

Divide the equation by your new-found root and find the other two from your quadratic.

Hope I helped.
 

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