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Finding relative extrema (first derivative test)

AP.CALC:
FUN‑4 (EU)
,
FUN‑4.A (LO)
,
FUN‑4.A.2 (EK)
Once you find a critical point, how can you tell if it is a minimum, maximum or neither? Created by Sal Khan.

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  • piceratops ultimate style avatar for user Min ChanHong
    what does local extrema mean?
    (14 votes)
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  • blobby green style avatar for user Gabby Stillman
    Is it possible to have two local maximums?
    (10 votes)
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  • blobby green style avatar for user Chris Kim
    Are all critical points maximums/minimums and vice versa? Is this the definition of a critical point (max/min)? Thank you.
    (6 votes)
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    • leafers ultimate style avatar for user KrisSKing
      All maximums and minimums are critical points, but it does NOT work the other way around. You can have a critical point that is not a maximum or minimum. In this video the point at x sub 3 is a critical point, but it is NOT a maximum nor minimum. This point is called an inflection point, and future videos explain inflection points.
      (13 votes)
  • blobby green style avatar for user Grayson Swaim
    Could there be more than 1 local maximums or local minimums per function? How about more than 1 global maximums and global minimums?
    (3 votes)
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    • leaf green style avatar for user clara
      You can only have one value for the absolute max or min, (like f(x)=1) but I think that value can appear more than once, as it does in the sine function. In f(x)=sin(x), I think 1 counts as the absolute max even though it occurs at pi/2, 5pi/2, 9pi/2, etc.
      (I think global and absolute maximums are the same thing, but absolute is what I studied.)
      (7 votes)
  • duskpin sapling style avatar for user Rohini
    What is an inflection point? Is it a critical point?
    (3 votes)
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  • primosaur ultimate style avatar for user Ephraim
    This whole video seems useless to me because to see if the surrounding derivative is positive or negative you have to look at the graph, but then you might as well see if its the greatest (or least) point in the area?
    (3 votes)
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    • piceratops ultimate style avatar for user Just Keith
      You cannot always tell from the graph. You might get a rough approximation of where the max or min is, but you need the derivative to check the exact value. Also, if you have max and min quite close to each other the graph can look like it has fewer max and min than it really has -- so you need the derivative to be sure. For example, could you tell just by looking at this graph how many maxima and minima there are: http://www.wolframalpha.com/share/clip?f=d41d8cd98f00b204e9800998ecf8427emr9u8s45l4

      Answer: There are 8.
      (4 votes)
  • female robot grace style avatar for user Angelica Willis
    In order for a point to be maximum or minimum, must the derivatives approaching from both sides be symmetrical? I mean, does the derivative of points leading up to the critical point from the left have to correspond with its additive inverse on the right?
    (4 votes)
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    • leafers ultimate style avatar for user KrisSKing
      No, the derivatives approaching from either side of the maximum or minimum do not have to be symmetrical. Try graphing the function y = x^3 + 2x^2 + .2x. You have a local maximum and minimum in the interval x = -1 to x = about .25. By looking at the graph you can see that the change in slope to the left of the maximum is steeper than to the right of the maximum. And the change in slope to the left of the minimum is less steep than that to the right of the minimum.
      (2 votes)
  • leaf blue style avatar for user Touseeq Hyder Abidi
    How can we calculate whether the critical number is either maxima or minima without graph?
    (3 votes)
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  • blobby green style avatar for user Joshua Sarp
    what is the trick in identifying a local minimum or maximum by looking at the graph?
    (3 votes)
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    • male robot hal style avatar for user Sid
      If the graph is descending and then starts ascending, you're at a local minimum where the transition occurs.

      With that information, you can probably figure out how to spot a local maximum.
      (3 votes)
  • leafers ultimate style avatar for user Andrew Escobedo
    Quick question, are the endpoints of a closed interval critical points?
    (2 votes)
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Video transcript

In the last video we saw that if a function takes on a minimum or maximum value, min max value for our function at x equals a, then a is a critical point. But then we saw that the other way around isn't necessarily true. x equal a being a critical point does not necessarily mean that the function takes on a minimum or maximum value at that point. So what we're going to try to do this video is try to come up with some criteria, especially involving the derivative of the function around x equals a, to figure out if it is a minimum or a maximum point. So let's look at what we saw in the last video. We saw that this point right over here is where the function takes on a maximum value. So this critical point in particular was x naught. What made it a critical point was that the derivative is 0. You have a critical point where either the derivative is 0 or the derivative is undefined. So this is a critical point. And let's explore what the derivative is doing as we approach that point. So in order for this to be a maximum point, the function is increasing as we approach it. The function is increasing is another way of saying that the slope is positive. The slope is changing but it stays positive the whole time which means that the function is increasing. And the slope being positive is another way of saying that the derivative is greater than 0 as we approach that point. Now what happens as we pass that point? Right at that point the slope is 0. But then as we past that point, what has to happen in order for that to be a maximum point? Well the value of the function has to go down. If the value of the function is going down, that means the slope is negative. And that's so another way of saying that the derivative is negative. So that seems like a pretty good criteria for identifying whether a critical point is a maximum point. So let's say that we have critical point a. We are at a maximum point if f prime of x switches signs from positive to negative as we cross x equals a. That's exactly what happened right over here. Let's make sure it happened at our other maximum point right over here. So right over here, as we approach that point the function is increasing. The function increasing means that the slope is positive. It's a different positive slope. The slope is changing, it's actually getting more, and more, and more steep. Or more, and more, and more positive. But is definitely positive. So it's positive going into that point. And then it becomes negative after we cross that point. The slope was undefined right at the point. But it did switch signs from positive to negative as we crossed that critical point. So these both meet our criteria for being a maximum point. So, so far our criteria seems pretty good. Now let's make sure that somehow this point right over here, which we identified in the last video as a critical point, let's make-- and I think we called this x0, this was x1, this was x2. So this is x3. Let's make sure that this doesn't somehow meet the criteria because we see visually that this is not a maximum point. So as we approach this, our slope is negative. And then as we cross it, our slope is still negative, we're still decreasing. So we haven't switched signs. So this does not meet our criteria, which is good. Now let's come up with the criteria for a minimum point. And I think you could see where this is likely to go. Well we identified in the last video that this right over here is a minimum point. We can see that. It's a local minimum just by looking at it. And what's the slope of doing as we approach it? So the function is decreasing, the slope is negative as we approach it. f prime of x is less than 0 as we approach that point. And then right after we cross it-- this wouldn't be a minimum point if the function were to keep decreasing somehow. The function needs to increase now. So let me do that same green. So right after that, the function starts increasing again. f prime of x is greater than 0. So this seems like pretty good criteria for a minimum point. f prime of x switches signs from negative to positive as we cross a. If we have some critical point a, the function takes on a minimum value at a if the derivative of our function switches signs from negative to positive as we cross a, from negative to positive. Now once again, this point right over here, this critical point x sub 3 does not meet that criteria. We go from negative to 0 right at that point then go to negative again. So this is not a minimum or a maximum point.