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

In summary, to find a cubic function g(x) that has a local maximum value of 3 at -7 and a local minimum value of -9 at 12, you need to find the values of a, b, c, and d. This can be done by setting the derivative of g(x) equal to zero and using the given points to solve for the coefficients. It is also important to consider the second derivative of g(x) to determine whether the points are local maxima or minima. Careful notation should be used when solving for the coefficients.
  • #1
ryan.1015
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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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  • #2
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:
  • #3
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.
 

1. What is a cubic function?

A cubic function is a polynomial function of degree three, meaning that it has a highest exponent of three. It can be written in the form of f(x) = ax^3 + bx^2 + cx + d, where a, b, c, and d are constants and x is the independent variable.

2. How do I find the roots of a cubic function?

The roots of a cubic function can be found by factoring the function or by using the cubic formula. Factoring involves finding common factors and using the quadratic formula to solve for the remaining roots. The cubic formula is a more complex formula that gives the exact values of the roots.

3. What is the process for graphing a cubic function?

To graph a cubic function, you can follow a few steps. First, determine the x and y-intercepts by setting x or y equal to zero. Then, plot these points on a graph. Next, find the vertex of the function by using the formula x = -b/2a, where a and b are the coefficients of the x^2 and x terms. Plot this point on the graph. Finally, use the shape of the function (whether it opens up or down) to plot additional points and create a smooth curve.

4. How can I identify the end behavior of a cubic function?

The end behavior of a cubic function can be identified by looking at the leading coefficient and the degree of the function. If the leading coefficient is positive and the degree is odd, the function will have a positive end behavior. If the leading coefficient is negative and the degree is odd, the function will have a negative end behavior.

5. Can a cubic function have more than three roots?

No, a cubic function can only have a maximum of three roots. This is because a cubic function is a polynomial of degree three, meaning it can only have three terms with the highest exponent of three. However, some of the roots may be repeated, resulting in less than three unique roots.

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