HELP Detrivative matrix/system of equations

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Discussion Overview

The discussion revolves around solving a problem involving a cubic polynomial and its derivative, specifically finding the polynomial given certain conditions. The scope includes mathematical reasoning and problem-solving techniques related to polynomial equations and derivatives.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant expresses difficulty in solving for the cubic polynomial p(x) given the conditions p(1) = 8, p(−1) = −3, p′(2) = 0, and p′(−2) = 0.
  • Another participant suggests starting with the general form of the polynomial p(x) = ax³ + bx² + cx + d and using the given conditions to form equations.
  • A participant clarifies that the problem is not for homework but for exam preparation, seeking to understand the solution process rather than just the answer.
  • A later reply provides a detailed breakdown of the equations derived from the conditions and suggests using Gaussian elimination or other methods to solve the system.
  • The same reply also simplifies the problem by determining that b = 0, which leads to further simplifications in finding the values of a, c, and d.

Areas of Agreement / Disagreement

Participants generally agree on the approach of forming equations from the given conditions, but there is no consensus on the best method for solving the system of equations, as different strategies are suggested.

Contextual Notes

Participants note that there are multiple methods for solving systems of equations, and the discussion reflects various approaches without resolving which is the most effective.

theacerf1
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Iv been stuck on this question for hrs now, no idea how to solve it. Can someone please explain, thanks! :)

Let p(x) be a cubic polynomial and p′(x) be its derivative, and suppose that p(1) = 8,
p(−1) = −3, p′(2) = 0 and p′(−2) = 0. Find p(x).
 
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Hello and welcome to the forum! First of all, if this is a homework question, you should post it in the homework section of the forum.

If this isn't a homework question, I'll be happy to post my solution, but I'll wait for your response. You could start by writing p(x) = ax3 + bx2 + cx + d and trying to solve for the unknowns using your restraints.
 
Hey spamiam, no its not a homework question, iv got my exam nxt week for math and just working through different textbooks solving different problems. The textbook only gives the answer but what I want to know is how they got the answer, basically the working out as i have no idea how to solve this kind of question if it pops up in the exam :(

Yeah, wat u wrote is all i could basically think of too...
p(x) = ax3 + bx2 + cx + d
p'(x) = 3ax^2 + 2bx + c

then subbing p(1) = 8, p(−1) = −3, p′(2) = 0 and p′(−2) = 0 to get 4 equations. Then I am totally stuck. There is more questions just like this so if i can work out exactly how to do this, i can attempt the other ones with different numbers.

Thanks for the help and quick reply :)
 
Well that's a good start: 4 equations with 4 unknowns. Here's what I got

(1) a + b + c + d = p(1) = 8

(2) -a + b - c + d = p(-1) = -3

(3) 12a + 4b + c = p'(2) = 0

(4) 12a - 4b + c = p'(-2) = 0

You can just use Gaussian elimination, but I think it might be easier to just solve this particular system using a couple tricks. Subtracting (4) from (3), I get 8b = 0, so b=0, simplifying things significantly. Adding (1) and (2) together, I get 2b + 2d = 5, so b + d = 5/2. Since b=0, then d = 5/2. Using (3) with b=0, we have c = -12a. Subtracting (2) from (1), we have 2a + 2c = 11, so a + c = 11/2. Substituting in c = -12a, we get a = -1/2, so c = 6. so [itex]p(x) = -\frac{1}{2}x^3 +6x + \frac{5}{2}[/itex].

Anyway, there are no real fixed rules for solving systems of equations, aside from Gaussian elimination, so you just have to play around with them to eliminate variables.
 

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