How can I find a cubic function with specific local maximum and minimum values?

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SUMMARY

The discussion focuses on finding a cubic function g(x) = ax^3 + bx^2 + cx + d that has a local maximum of 3 at x = -7 and a local minimum of -9 at x = 12. The derivative g'(x) must equal zero at these critical points, leading to the equation g'(x) = A(x + 7)(x - 12). The coefficients were derived as A = 1/3, b = 5/2, and c = -84, but the constant d remains to be determined. The second derivative g''(x) is also relevant for confirming the nature of the critical points.

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  • Calculate the constant d in the cubic function g(x) using the known values g(-7) = 3 and g(12) = -9
  • Explore the implications of the second derivative test for local extrema
  • Investigate the relationship between the coefficients of the derivative and the original cubic function
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Homework Statement


find a cubic function g(x)=ax^3 +bx^2+cx +d that has a local maximum value of 3 at -7 and a local minimum value 0f -9 at 12.


Homework Equations





The Attempt at a Solution


I know the derivative should equal zero for a max or min to occure. So i got f '(x)=(x+7)(x-12). then i got F '(x)=x^2-5x-84 and plugged that into the original equation. I got A=1/3 b=5/2 and c=-84. I'm not sure how to fet D, or if i did this first part right
 
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Basically you know that g(3)=-7 and g(-9)=12. And also that g'(3)=g'(9)=0. Solve now.


EDIT: It should be g(7)=-3 and g(12)=9, not the other way around
 
Last edited:
You switch function names, with the original starting out as g(x).

g'(x) doesn't have to be exactly (x + 7)(x - 12). It could be a constant multiple of this expression, namely g'(x) = A(x + 7)(x - 12) = A(x^2 - 5x - 84). Also, you can calculate g'(x) from the original equation for g, and compare this to the one above.

You know that g(-7) = 3 and that g(12) = -9.

What about the second derivative? You can calculate g''(x) from the equation above, as well as from the original equation. What do you know about the value of the second derivative at a local maximum? At a local minimum?

You should be more careful with your notation. You have referred to the original function as g, f, and F. Also, the coefficients of the equation for g(x) involved a, b, c, and d, not A.
 

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