Identifying and Classifying Stationary Points for f(x) = x^3 + ax^2 + 5x + 2

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In summary, to find the other stationary point for the function f(x) = x^3 +ax^2 +5x +2, we can set the first derivative, f'(x), equal to zero and solve for a. Then, we can substitute this value of a into the equation and solve for x to find the other stationary point. To classify the stationary points, we can use the second derivative test by plugging in the x-values of the stationary points into the second derivative, f''(x), and determining the sign of the result.
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Carl_M
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Homework Statement



f(x) = x^3 +ax^2 +5x +2 and has two stationary points, one of which is x = 2. a) Find the other stationary point.
b) classify the stationary points using the second derivative test

The Attempt at a Solution



f '(x) = 2ax + 3x^2 +5
f ''(x) = 2a + 6x

0 = (2)^3 + a(2^2) +5(2) +2
0 = 8 + 4a + 10 +2
0 = 20 + 4a
-4a = 20, a = -5

What am I supposed to do to get a stationary point?
 
Last edited:
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  • #2
0 = (2)^3 + a(2^2) +5(2) +2

For a stationary point, f'(x) = 0, not f(x) = 0.
 
  • #3
f ' (x) = 0
So that would be?
2ax + 3x^2 + 5 = 0
How would I find a?

Would I use 0 = (2)^3 + a(2^2) +5(2) +2 to get a = -5

0 = 2(-5)x + 3x^2 +5
0 = -10x + 3x^2 +5
0 = 3x^2 - 10x + 5
x = (5 +sqrt(10))/3 and (5 - sqrt(10))/3 ?

or

0 = ( 2a(2) + 3(2)^2 +5
0 = 4a + 12 + 5
0 = 4a +17
-4a = 17
a = -17 / 4

substitute -17/4 into 2ax + 3x^2 + 5 = 0?
0 = 2(-17/4)x + 3x^2 + 5 = 0
0 = 3x^2 - 8.5x + 5 = 0
x = 2 , 5/6

the second derivative?
 
Last edited:
  • #4
You have to substitute x=2 in the entire equation, not just one variable.
 

Related to Identifying and Classifying Stationary Points for f(x) = x^3 + ax^2 + 5x + 2

1. What is a stationary point?

A stationary point is a point on a graph where the derivative or slope of the function is equal to zero. This means that the graph is neither increasing nor decreasing at that point.

2. Why is finding stationary points important?

Finding stationary points helps us identify critical points on a graph, which can be used to determine the maximum and minimum values of a function. This is important in many fields of science, including physics, economics, and engineering.

3. How do you find stationary points?

To find stationary points, you need to take the derivative of the function and set it equal to zero. Then, solve for the variable to determine the x-values of the stationary points. You can also use a graphing calculator to visually identify the stationary points.

4. What do the different types of stationary points indicate?

There are three types of stationary points: maximum points, minimum points, and points of inflection. Maximum points indicate the highest point on a graph, while minimum points indicate the lowest point. Points of inflection indicate a change in the concavity of the graph.

5. Can there be more than one stationary point on a graph?

Yes, there can be multiple stationary points on a graph. In fact, a function can have an infinite number of stationary points. It is important to consider the context of the problem to determine which stationary point is the most relevant.

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