# Calculus

posted by Sean

For any constant c, define the function f_c(x)= x^3+2x^2+cx.
(a) Graph y = f_c(x) for these values of the parameter c: c = -1, 0, 1, 2, 3, 4. What are the similarities and differences among the graphs, and how do the graphs change as the parameter increases?
(b) For what values of the parameter c will f_c have one local maximum and one local minimum? Use Calculus. As c increases, what happens to the distance between the local maximum and the local minimum?
(c) For what values of the parameter c will f_c have no local maximum or local minimum? Use calculus.
(d) Are there any values of the parameter c for which f_c will have exactly one horizontal tangent line?

1. MathMate

If f(x)=x^3+2x^2+cx.
Let f'(x)=3x^2+4x+c be the derivative

b. one minimum and one maximum
To have one maximum and one minimum, f'(x)=0 must have distinct roots. For this to happen, the discriminant, Δ, of the quadratic formula must be >0, or b²-4ac > 0. Note: the c in this expression is the constant term of f'(x), which also happens to be c.

c. no min. no max.
In order to have no local maximum and local minimum, then f'(x)=0 should have no real roots. The quadratic will have two complex roots if Δ is less than zero, or b²-4ac < 0.

d. exactly one horizontal line
This will happen when there are two coincident roots. The two roots will be merged together when Δ=0, or b²-4ac=0.
Solve for the value of c (the constant).
At for the value of c where Δ=0, find the value of x0 where f'(x0)=0.
Confirm that f"(x0)=0 which implies a point of inflexion. I got x0=-2/3.

2. Sean

So what does the derivative of the f(x) have to do with the graphs? Or is the derivative part of the main question: define the function f with the formula? And what would be the similarities and differences of the graphs? And if you increase c then does the slope change?

3. MathMate

a.
The question want you to
1. graph the function f(x) (NOT f'(x)) for the given values of the parameter.
2. Describe the similarities and differences of the graphs.
3. Describe the effect of the value of c with respect to the appearances of the graphs (of f(x)).

The derivatives part comes later in b,c and d.

Do you have a graphing calculator?

4. Sean

What kind of similarities and differences is it asking for?

5. MathMate

If you have graphed the function for the different values of c requested, you will see the differences.
In fact, part a is to help you understand the questions in parts b, c and d.
If you have not graphed the function as requested, you can also read up parts b, c, and d for the different possible cases when c is varied.

6. Sean

So then there are no similarities? And the differences are the shapes and the distinct roots, max and min?

7. MathMate

I would expect similarities for larger values of c. The discriminant equals zero at c=4/3. So I expect similarities when c>2. For c≤1, I expect changes in form (max-min, inflexion, etc.)
I do not have a graphing calculator, so I cannot tell you the differences and similarities offhand.

8. Sean

Okay thank you very much

9. MathMate

You're welcome!

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