Critical Points definition

In summary: Bear explains that a "critical point" is where the derivative either does not exist or is 0, while an "inflection point" is where the second derivative changes sign, which can only happen where the second derivative is 0 but the first is not necessarily. Examples are given to clarify this relationship. In summary, a critical point must be in the domain of the function and have a first derivative of 0 or undefined, while an inflection point may also be a critical point but not always.
  • #1
daivinhtran
68
0
Are inflection points critical points?

and what about at the value that f(x) undefined? Is that critical point too?
 
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  • #2
A "critical point" is a point at which the derivative either does not exist or is 0. An "inflection point" is where the second derivative changes sign. Obviously, that can only happen where the second derivative is 0 but the other way is not true. For example, [itex]y= x^4[/itex] has [itex]y'= 4x^3[/itex] which is 0 only for x= 0 but the second derivative is [itex]y''= 12x^2[/itex] which is 0 at x= 0 but does NOT change sign there.

On the other hand, [itex]y= x^3+ x[/itex] has [itex]y'= 3x^2+ 1[/itex] and [itex]y''= 6x[/itex]. At x= 0, y'' changes sign but the first derivative is not 0 there. x= 0 is an inflection point but is NOT critical point.

For you last question, no. f(x) must be defined at a critical point.
 
  • #3
daivinhtran said:
Are inflection points critical points?

and what about at the value that f(x) undefined? Is that critical point too?

To be a critical point:
- It must be in the domain of the function, meaning the function must be defined at the point.
- The first derivative must either be 0 or undefined.

Some inflection points are also critical points, but not all inflection points are critical.

BiP
 

1. What is a critical point?

A critical point is a point on a function where the derivative is equal to zero or undefined. It can also be defined as a point where the tangent line is horizontal or the slope of the function is changing.

2. How do you find critical points?

To find critical points, you need to take the derivative of the function and set it equal to zero. Then, solve for the variable to find the x-values of the critical points. You can also use the second derivative test to confirm if the critical points are maximum, minimum, or inflection points.

3. What is the significance of critical points?

Critical points are important in calculus because they represent the locations where the behavior of a function changes. These points can help us identify maximum and minimum values of a function and determine if the function is increasing or decreasing in a specific interval.

4. Can a function have more than one critical point?

Yes, a function can have multiple critical points. This can happen when the function has multiple peaks and valleys, or when there are multiple points where the slope changes from positive to negative or vice versa.

5. Are all critical points local extrema?

No, not all critical points are local extrema. Some critical points can be inflection points, where the function changes from being concave up to concave down or vice versa. To determine if a critical point is a local extrema, we need to use the second derivative test or check the behavior of the function in the surrounding intervals.

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