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# 2011 Calculus AB free response #4c

Finding the points of inflection for a strangely defined function. Created by Sal Khan.

## Want to join the conversation?

• Wouldn't the point (-3, 0) also be a point of inflection, because the rate of change of the slope at that point changes from positive to negative? Just visually looking at the graph leads me to believe this must be the case.
• (-3, 0) is a point of inflection for f(x), but not for g(x). In this problem, the graph shown is the derivative of g(x), so to find inflection of g(x) from this graph, we have to see where the slope of this graph goes from positive to negative (or vice versa). I think you're thinking this graph represents g(x), when it is actually g'(x).
• My textbook says that there's more than one definition of inflection point and that the one they use in the book requires that there exists a tangent line and the tangent line intersects the graph. According to my book's definition, would the answer be that g has no inflection points?.
• Yes. This is because this is an odd function, that, like Sal said, is not differentiable at f(0) because the slope jumps. So the book's definition doesn't work for this strange problem.
• I'm a little confused on this. Since f '(x) does not exist at this point, then does this not imply that g ' ' (x) also does not exist? So, therefore, the function has no inflection point?
• Generally... an inflection point is understood to be a point where the concavity/acceleration changes from positive to negative or from negative to positive.

It does not matter whether or not a function's output value exists.
In this case, we just want a point where f'(x) changes between positive and negative.
• In order for a given point to qualify as an Inflection Point of a function f, must the second derivative of f (with respect to x) be Continuous at said point? Or is it simply enough for the function to be convex on one side and concave on the other (and with a limit that exists at that point)?
(1 vote)
• A point by definition is an inflection point if the concavity changes at that point(from concave up to concave down or vice versa). f does not have to be continuous or differentiable at that point. Nor does the limit have to exist there.
(1 vote)
• I've been led to believe that any justification on the AP Calculus AB Exam requires us to use the given information. If that's the case, this would be my justification:

"x = 0 is an extremum point of f(x) and thus an extremum point of f(x) + 2 = g'(x).

x = 0 is, therefore, an inflection point of g(x)."

Would this be okay?
(1 vote)

## Video transcript

Part C. Find all values of x on the interval negative 4 is less than x is less than 3 for which the graph of g has a point of inflection. Give a reason for your answer. So an inflection point is a point where the sign of the second derivative changes. So if you take the second derivative at that point, or as we go close to that point, or as we cross that point, it goes from positive to negative or negative to positive. And to think about that visually, you could think of some examples. So if you have a curve that looks something like this, you'll notice that over here the slope is negative, but it's increasing. It's getting less negative, less negative. Then it goes to 0. Then it keeps increasing. Slope is increasing, increasing, all the way to there, and then it starts getting less positive. So it starts decreasing. So it's increasing. The slope is increasing over at this point right over here. So even though the slope is negative, it's getting less negative over here. So it's increasing. And then the slope keeps increasing. It keeps on getting more and more positive up to about this point. And then the slope is positive, but then it becomes less positive. So the slope begins decreasing after that. So this right over here is a point of inflection. The slope has gone from increasing to decreasing. And if the other thing happened, if the slope went from decreasing to increasing, that would also be a point of inflection. So if this was maybe some type of a trigonometric curve, then you might see something like this. And so this also would it be a point of inflection. But for this, our g of x is kind of hard to visualize the way they've defined it right over here. So the best way to think about it is just figure out where its second derivative has a sign change. And to think about that, we have to find its second derivative. So let's write g of x over here. We know g of x is equal to 2x plus the definite integral from 0 to x of f of t dt. We've already taken its derivative, but we'll do it again. g prime of x is equal to 2 plus-- fundamental theorem of calculus. The derivative of this right over here is just f of x. And if we have the second derivative of g-- g prime prime of x-- this is equal to-- derivative of 2 is just 0. And then derivative of f of x is f prime of x. So asking where this has a sign change, asking where our second derivative has a sign change, is equivalent to asking where does the first derivative of f have a sign change? And asking where the first derivative of f has a sign change is equivalent to saying where does the slope of f have a sign change? You can view this as the slope or the instantaneous slope of f. So we want to know when the slope of f has a sign change. So let's think about. Over here, the slope is positive. It's going up. It's up. It's increasing, but it's positive. And that's what we care about. So let's write it. I'll do it in green. So the slope is positive this entire time. It's increasing. It's increasing. It's positive. It's getting less positive now. It's starting to decrease, but the slope is still positive. The slope is still positive all the way until we get right over there. You can see it gets pretty close to zero. And then the slope gets a negative. And then right over here, the slope is negative. The slope is negative right over here. So this is interesting. Because even though f is actually not differentiable right here-- so f is not differentiable at that point right over there. And you could see it because the slope goes pretty close to 0, and then it just jumps to negative 3. So you have a discontinuity of the derivative right over there, but we do have a sign change. We go from having a positive slope on this part of the curve to having a negative slope over this part of the curve. So we experience a sign change right over here at x is equal to 0, a sign change in the first derivative of f, which is the same thing as saying a sign change in the second derivative of g. And a sign change in the second derivative of g tells us that, when x is equal to 0, the graph of g has a point of inflection.