Relative Extrema: Find a,b,c,d for f(x)

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Homework Help Overview

The problem involves finding the coefficients a, b, c, and d for a cubic function f(x) = ax³ + bx² + cx + d, given that the function has relative extrema at the points (1,2) and (2,3). The discussion centers around the implications of these critical points on the derivative of the function.

Discussion Character

  • Exploratory, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the relationship between the critical points and the derivative of the function, noting that f'(x) should equal zero at x=1 and x=2. There is an exploration of how to set up equations based on these conditions and how to relate them to solve for the unknown coefficients.

Discussion Status

Some participants have suggested substituting the critical points into the derivative to form a system of equations. There is an acknowledgment of the need for additional equations to solve for all unknowns, indicating that the discussion is ongoing and participants are seeking further insights or information to progress.

Contextual Notes

Participants are working under the constraint of needing to find four unknowns (a, b, c, d) with only two equations derived from the critical points, leading to a challenge in determining a complete solution.

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Homework Statement


I have to find a, b, c, and d such that the function defined by :
f(x) = ax3+bx2+cx+d
will have a relative extrema at points (1,2) and (2,3).

The Attempt at a Solution


From the given critical points, I am able to know that when x=1 or x=2, f'(x)=3ax2+2bx+c should be equal to zero.

Therefore f'(x) should have factors like (x-1) and (x-2) or in simplified form x2-3x+2. Now my problem is that how should I relate the two equations of f'(x) for me to be able to solve for the said unknowns?
 
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Well you know f'(x) = 3ax^2 + 2bx + c = 0 when x=1 and x=2, So substitute that in. You will get two equations which you can view as linear equations in the variables a, b, c, d, with the coefficient of d being zero.

What else do you need to solve this system and how can you get that information?
 
Gib Z said:
Well you know f'(x) = 3ax^2 + 2bx + c = 0 when x=1 and x=2, So substitute that in. You will get two equations which you can view as linear equations in the variables a, b, c, d, with the coefficient of d being zero.

What else do you need to solve this system and how can you get that information?

>>> after getting the two equations: 3a + 2b + c and 12a + 4b + c, what will I do... I still could not find a way to get the values of a, b, c, and d... after eliminating c, I only get 9a + 2b giving two variables unknown still.. help.. :(
 
You can get two more equations. Look at all your information again!
 

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