Solving 4th Order Diff. Eq. with Complex Root: Daunting Task?

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Discussion Overview

The discussion revolves around solving a fourth-order differential equation with complex roots. Participants explore the process of finding the homogeneous solution, particularly focusing on the implications of having complex roots and the necessary steps to derive the solution.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in solving the homogeneous part of a fourth-order differential equation due to the presence of a complex root (-2 + 3i).
  • Another participant clarifies that the real coefficients imply there are actually two complex roots: -2 + 3i and -2 - 3i.
  • A suggestion is made to consider the quadratic factor that arises from the complex roots, referencing the method used for quadratic equations with complex roots.
  • A later reply indicates that the participant successfully found the quadratic equation associated with the complex roots, although the details of the process are not fully elaborated.
  • One participant provides a detailed explanation of how to construct the quadratic factor from the complex roots and suggests dividing the original polynomial by this factor to find the other roots.

Areas of Agreement / Disagreement

Participants generally agree on the nature of the roots and the method to find the quadratic factor, but there is no consensus on the specific steps taken to reach the solution, as one participant initially struggled with the process.

Contextual Notes

Some limitations include the initial participant's lack of formal training in polynomial roots, which may affect their understanding of the problem. Additionally, the discussion does not resolve the complete solution to the fourth-order differential equation.

Who May Find This Useful

This discussion may be useful for students or individuals interested in differential equations, particularly those dealing with higher-order equations and complex roots.

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I have a 4th order differential equation with given -2 +3i root.

Now need to find the homogenous solution. Well, if the root was real, it would be easier but now I'm stuck and don't know how to proceed.

What am I supposed to do to solve this ?

Equation is : d4y(t)/dt4 +6d3y(t)/dt3 + 22d2y(t)/dt2 + 30dy(t)/dt + 13y(t) = f(t)

Just need to solve the homogenous part so f(t) is just a dummy function
 
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The coefficients are real, so actually you have been given two complex roots, not one.

If you haven't done any formal courses about roots of polynomials, think about what you get when you solve a quadratic equation with a pair of complex roots. That should lead you to finding a quadratic factor (with real coefficients) of the 4th-order equation.
 
AlephZero said:
The coefficients are real, so actually you have been given two complex roots, not one.

If you haven't done any formal courses about roots of polynomials, think about what you get when you solve a quadratic equation with a pair of complex roots. That should lead you to finding a quadratic factor (with real coefficients) of the 4th-order equation.

You're right there are two roots are given -2 - 3i and -2 + 3i
I tried to recreate a quadratic equation with those roots by assuming coefficient of a=1, b=4 and c=13. However it didn't work.
 
Ok, I've just done it, thanks though.
 
Great! For those who are interested, though, let me note that since [itex]-2+3i[/itex] and [itex]-2-3i[/itex] are roots then [itex](x- (-2+3i))(x-(-2-3i))= (x+2- 3i)(x+2+ 3i)= (x+2)^2- (3i)^2= x^2+ 4x+ 4+ 9= x^2+ 4x+ 13[/itex]. Now divide [itex]x^4+ 6x^3+ 22x^2+ 30x+ 13[/itex] by that to find the quadratic equation the other roots must satisfy.
 

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