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Worked example: Inflection points from first derivative

Sal analyzes the graph of a the derivative g' of function g to find all the inflection points of g.

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• At , you said that the correct answer is x=-3 but isn't this the graph of g'(x)?? The question is asking for the left most point of inflection in the graph of g(x) not g'(x)
• What's the actually ''leftmost'' means ?
• It means furthest to the left. For instance the leftmost letter in "ERKUT" is 'E'
• Before the point x= -3, if the function was increasing suddenly how it can become 0 at x = -3, Is it not decreasing there ?
(1 vote)
• The graph is of the derivative not the original function ...

• What do we call the plateaus in a f ' (x) or first derivative curve (places where the curve briefly goes flat relative to the x axis similarly to a horizontal tangent of the f (x) curve but without creating a max or min point)? Are they inflection points? Do they provide significant data or information? I would assume not since Sal had not devoted a section to them, but I'm curious.
(1 vote)
• These areas are not places where an inflection point exists because the derivative function changes its sign (it's neither increasing nor decreasing).
• Can we still tell if there is an inflection point if there is a node in the first derivative?
(1 vote)
• So you are saying that if on a place where is inflection point supposted to be is function not defined? In that case no, we cant tell there is inflection point, when its not here.
But I am not certain if this is what you asked, try to formulate your questions more precise.
• So the change in direction of the curves define where the inflection points would be on the original problem?
(1 vote)
• Let f' is not differentiable at x = c (i.e, f" doesn't exist), and f' changes its character from increasing to decreasing or vice versa at x = c. Do we have an inflection point at x = c?
(1 vote)
• Yes. Similar to how the first derivative need not exist at a critical point, the second derivative need not exist at an inflection point.
(1 vote)
• An inflection point is where the curve of the graph goes from concave down to up or vice versa. However the points sal highlighted were where the slope is zero but doesnt change concavity. This whole video looks wrong to me, can anyone point me in the right direction?
(1 vote)
• Would there be an inflection point if the first derivative graph went from a slope of zero to a positive or negative slope or only between changing non-zero slopes? Also would there be one if the slope was vertical at some point?
(1 vote)
• An inflection point has both first and second derivative values equaling zero. For a vertical tangent or slope , the first derivative would be undefined, not zero. For a transition from positive to negative slope values without the value of the slope equaling zero between them , the first derivative must have a discontinuous graph. In both these cases, the points would not be inflection points.
(1 vote)

Video transcript

- [Voiceover] So we're told let g be a differentiable function defined over the closed interval from negative six to six. The graph of its derivative, so they're giving the graphing the derivative of g, g prime is given below. So this isn't the graph of g. This is the graph of g prime. What is the x value of the left-most inflection point, inflection point in the graph of g. So they want, they don't want to know the x value of the inflection points in the graph of g prime, in this graph. They want to know the inflection points, the x values of the inflection points, in the graph of g. And we have to figure out the left-most one. So, let me just make a little table here, to think about what is happening at inflection points in our second derivative, our first derivative, and our actual function. So, this is g prime, prime. This is g prime. And this is our actual, I guess you could call it, the original function. So an inflection point are points where our second derivative is switching sides. It's going from positive negative or negative to positive. So, let's consider that first scenario. So g, so going from positive to negative. Positive, positive, to negative. So if g prime prime, if the second derivative's going from positive to negative, what is the first derivative doing? Well, remember, the second derivative, is the derivative of the first derivative. So, where the second derivative is positive, where the second derivative is positive, that means that the first derivative is increasing. So, if second derivative's going from positive to negative, that means first derivative is going from increasing to decreasing. From increasing to decreasing. And the function itself, well when the second derivative is positive, we are going to be, that means, that means that the slope is constantly increasing. And so that means that we are concave upwards. So, concave upwards. Upwards to downwards. To concave, to concave downwards. But they've given us the graph of g prime. So let's focus on what are the points where g prime is going from increasing to decreasing. So let's see. G prime is increasing, increasing, increasing, increasing, increasing at a slower rate, and then it starts decreasing. So, right over there it's going from increasing to decreasing. So then it's decreasing, decreasing, decreasing. Then it goes increasing, increasing, increasing, increasing, and then decreasing again. So that's another point where we're going from increasing to decreasing. And those are the only ones that look like we're going from increasing to decreasing. But we're not done yet. Because it's not just about going from the second derivative going from positive to negative, it's also the other way around. Any time the second derivative is switching signs. So, it's also the situation where we're going from negative to positive. Or, for the first derivative is going from decreasing, decreasing to increasing. Decreasing to increasing. Well let's see we're decreasing, decreasing, decreasing, and then we're increasing, alright. So it's right there. And then we're increasing, decreasing, decreasing, decreasing, and then we're increasing. So right over there. So these are the inflection points that I've just figured out visually. So, if you look at the choices, if we want to answer the original question, well the left-most one is it x is equal to negative three? It's x equals negative three. X equals negative one is indeed a x value, where we have an inflection point. And let's see, x equals two is one, and so is x equals four. So they actually listed, all of these are inflection points. And they just wanted the left-most one.