# second derivative concavity

Perhaps the easiest way to understand how to interpret the sign of the second derivative is to think about what it implies about the slope of â¦ If $$f'$$ is constant then the graph of $$f$$ is said to have no concavity. Conversely, if the graph is concave up or down, then the derivative is monotonic. The following theorem officially states something that is intuitive: if a critical value occurs in a region where a function $$f$$ is concave up, then that critical value must correspond to a â¦ Concavity and 2nd derivative test WHAT DOES fââ SAY ABOUT f ? That means as one looks at a concave up graph from left to right, the slopes of the tangent lines will be increasing. A function is concave down if its graph lies below its tangent lines. Setting $$S''(t)=0$$ and solving, we get $$t=\sqrt{4/3}\approx 1.16$$ (we ignore the negative value of $$t$$ since it does not lie in the domain of our function $$S$$). Note: Geometrically speaking, a function is concave up if its graph lies above its tangent lines. In the next section we combine all of this information to produce accurate sketches of functions. Figure $$\PageIndex{1}$$: A function $$f$$ with a concave up graph. Figure $$\PageIndex{13}$$: A graph of $$f(x)$$ in Example $$\PageIndex{4}$$. We find $$f''$$ is always defined, and is 0 only when $$x=0$$. CalculusQuestTM Version 1 All rights reserved---1996 William A. Bogley Robby Robson. Figure $$\PageIndex{12}$$: Demonstrating the fact that relative maxima occur when the graph is concave down and relatve minima occur when the graph is concave up. Subsection 3.6.3 Second Derivative â Concavity. To determine concavity without seeing the graph of the function, we need a test for finding intervals on which the derivative is increasing or decreasing. Concavity Using Derivatives You can easily find whether a function is concave up or down in an interval based on the sign of the second derivative of the function. If $$(c,f(c))$$ is a point of inflection on the graph of $$f$$, then either $$f''=0$$ or $$f''$$ is not defined at $$c$$. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. THeorem $$\PageIndex{3}$$: The Second Derivative Test. Figure $$\PageIndex{4}$$: A graph of a function with its inflection points marked. Such a point is called a point of inflection. ", "As the immunization program took hold, the rate of new infections decreased dramatically. If $$f''(c)>0$$, then the graph is concave up at a critical point $$c$$ and $$f'$$ itself is growing. Find the inflection points of $$f$$ and the intervals on which it is concave up/down. Our study of "nice" functions continues. Thus $$f''(c)>0$$ and $$f$$ is concave up on this interval. Thus the derivative is increasing! Consider Figure $$\PageIndex{1}$$, where a concave up graph is shown along with some tangent lines. Since $$f'(c)=0$$ and $$f'$$ is growing at $$c$$, then it must go from negative to positive at $$c$$. Sometimes, rather than using the first derivative test for extrema, the second derivative test can also help you to identify extrema. In other words, the graph of f is concave up. The second derivative gives us another way to test if a critical point is a local maximum or minimum. The derivative of a function f is a function that gives information about the slope of f. If the concavity of $$f$$ changes at a point $$(c,f(c))$$, then $$f'$$ is changing from increasing to decreasing (or, decreasing to increasing) at $$x=c$$. We technically cannot say that $$f$$ has a point of inflection at $$x=\pm1$$ as they are not part of the domain, but we must still consider these $$x$$-values to be important and will include them in our number line. Find the inflection points of $$f$$ and the intervals on which it is concave up/down. What does a "relative maximum of $$f'$$" mean? This is the point at which things first start looking up for the company. What is being said about the concavity of that function. If the second derivative of a function f(x) is defined on an interval (a,b) and f ''(x) > 0 on this interval, then the derivative of the derivative is positive. Inflection points indicate a change in concavity. If knowing where a graph is concave up/down is important, it makes sense that the places where the graph changes from one to the other is also important. This content is copyrighted by a Creative Commons Attribution - Noncommercial (BY-NC) License. The figure shows the graphs of two Let $$c$$ be a critical value of $$f$$ where $$f''(c)$$ is defined. We utilize this concept in the next example. Interval 2, $$(-1,0)$$: For any number $$c$$ in this interval, the term $$2c$$ in the numerator will be negative, the term $$(c^2+3)$$ in the numerator will be positive, and the term $$(c^2-1)^3$$ in the denominator will be negative. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. A the first derivative must change its slope (second derivative) in order to double back and cross 0 again. That means as one looks at a concave down graph from left to right, the slopes of the tangent lines will be decreasing. To show that the graphs above do in fact have concavity claimed above here is the graph again (blown up a little to make things clearer). The second derivative can be used to determine the concavity and inflection point of a function as well as minimum and maximum points. Find the point at which sales are decreasing at their greatest rate. The second derivative tells whether the curve is concave up or concave down at that point. The second derivative test for concavity states that: If the 2nd derivative is greater than zero, then the graph of the function is concave up. The first derivative of a function gave us a test to find if a critical value corresponded to a relative maximum, minimum, or neither. Figure $$\PageIndex{5}$$: A number line determining the concavity of $$f$$ in Example $$\PageIndex{1}$$. We have found intervals of increasing and decreasing, intervals where the graph is concave up and down, along with the locations of relative extrema and inflection points. Describe the concavity â¦ The graph of a function $$f$$ is concave up when $$f'$$ is increasing. Example $$\PageIndex{3}$$: Understanding inflection points. (1 vote) Ï 2-XL Ï When the graph is concave up, the critical point represents a local minimum; when the graph is concave down, the critical point represents a local maximum. We have identified the concepts of concavity and points of inflection. "Wall Street reacted to the latest report that the rate of inflation is slowing down. http://www.apexcalculus.com/. We can apply the results of the previous section and to find intervals on which a graph is concave up or down. Graphically, a function is concave up if its graph is curved with the opening upward (Figure 1a). When $$f''<0$$, $$f'$$ is decreasing. Since the concavity changes at $$x=0$$, the point $$(0,1)$$ is an inflection point. The second derivative is evaluated at each critical point. Evaluating $$f''$$ at $$x=10$$ gives $$0.1>0$$, so there is a local minimum at $$x=10$$. Time saving links below. Notice how the tangent line on the left is steep, upward, corresponding to a large value of $$f'$$. If a function is decreasing and concave up, then its rate of decrease is slowing; it is "leveling off." The second derivative $$f''(x)$$ tells us the rate at which the derivative changes. The number line in Figure $$\PageIndex{5}$$ illustrates the process of determining concavity; Figure $$\PageIndex{6}$$ shows a graph of $$f$$ and $$f''$$, confirming our results. Let $$f$$ be differentiable on an interval $$I$$. These results are confirmed in Figure $$\PageIndex{13}$$. We find $$S'(t)=4t^3-16t$$ and $$S''(t)=12t^2-16$$. Figure $$\PageIndex{8}$$: A graph of $$f(x)$$ and $$f''(x)$$ in Example $$\PageIndex{2}$$. The inflection points in this case are . On the right, the tangent line is steep, upward, corresponding to a large value of $$f'$$. The second derivative of a function f can be used to determine the concavity of the graph of f. A function whose second derivative is positive will be concave up (also referred to as convex), meaning that the tangent line will lie below the graph of the function. THeorem $$\PageIndex{2}$$: Points of Inflection. Concavity is simply which way the graph is curving - up or down. The Second Derivative Test The Second Derivative Test relates the concepts of critical points, extreme values, and concavity to give a very useful tool for determining whether a critical point on the graph of a function is a relative minimum or maximum. And where the concavity switches from up to down or down to up (like at A and B), you have an inflection point, and the second derivative there will (usually) be zero. A graph of $$S(t)$$ and $$S'(t)$$ is given in Figure $$\PageIndex{10}$$. Reading: Second Derivative and Concavity Graphically, a function is concave up if its graph is curved with the opening upward (figure 1a). Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. The graph of $$f$$ is concave up on $$I$$ if $$f'$$ is increasing. We conclude $$f$$ is concave down on $$(-\infty,-1)$$. When $$f''>0$$, $$f'$$ is increasing. Notice how the tangent line on the left is steep, downward, corresponding to a small value of $$f'$$. If the 2nd derivative is less than zero, then the graph of the function is concave down. Reading: Second Derivative and Concavity. On the interval of $$(1.16,2)$$, $$S$$ is decreasing but concave up, so the decline in sales is "leveling off.". A function whose second derivative is being discussed. Notice how $$f$$ is concave down precisely when $$f''(x)<0$$ and concave up when $$f''(x)>0$$. A function is concave down if its graph lies below its tangent lines. Figure $$\PageIndex{3}$$: Demonstrating the 4 ways that concavity interacts with increasing/decreasing, along with the relationships with the first and second derivatives. We conclude that $$f$$ is concave up on $$(-1,0)\cup(1,\infty)$$ and concave down on $$(-\infty,-1)\cup(0,1)$$. ". We find $$f'(x)=-100/x^2+1$$ and $$f''(x) = 200/x^3.$$ We set $$f'(x)=0$$ and solve for $$x$$ to find the critical values (note that f'\ is not defined at $$x=0$$, but neither is $$f$$ so this is not a critical value.) Evaluating $$f''(-10)=-0.1<0$$, determining a relative maximum at $$x=-10$$. Free companion worksheets. If the second derivative is positive at a point, the graph is bending upwards at that point. We were careful before to use terminology "possible point of inflection'' since we needed to check to see if the concavity changed. Our definition of concave up and concave down is given in terms of when the first derivative is increasing or decreasing. Figure $$\PageIndex{6}$$: A graph of $$f(x)$$ used in Example$$\PageIndex{1}$$, Example $$\PageIndex{2}$$: Finding intervals of concave up/down, inflection points. We find that $$f''$$ is not defined when $$x=\pm 1$$, for then the denominator of $$f''$$ is 0. Using the Quotient Rule and simplifying, we find, $f'(x)=\frac{-(1+x^2)}{(x^2-1)^2} \quad \text{and}\quad f''(x) = \frac{2x(x^2+3)}{(x^2-1)^3}.$. This calculus video tutorial provides a basic introduction into concavity and inflection points. Notice how $$f$$ is concave up whenever $$f''$$ is positive, and concave down when $$f''$$ is negative. Similarly, a function is concave down if its graph opens downward (Figure 1b). For instance, if $$f(x)=x^4$$, then $$f''(0)=0$$, but there is no change of concavity at 0 and also no inflection point there. Note that we need to compute and analyze the second derivative to understand concavity, so we may as well try to use the second derivative test for maxima and minima. So, as you can see, in the two upper graphs all of the tangent lines sketched in are all below the graph of the function and these are concave up. A point of inflection is a point on the graph of $$f$$ at which the concavity of $$f$$ changes. Pick any $$c<0$$; $$f''(c)<0$$ so $$f$$ is concave down on $$(-\infty,0)$$. Thus the derivative is increasing! Figure $$\PageIndex{7}$$: Number line for $$f$$ in Example $$\PageIndex{2}$$. Note: Geometrically speaking, a function is concave up if its graph lies above its tangent lines. Now consider a function which is concave down. Heinold of Mount Saint Mary 's University is the sign of the function has inflection... ( c\ ) =x^3-3x+1\ ) and is 0 only when \ ( f'\ ) is the point of maximum.... ) =0\ ) we can apply the same technique to \ ( \PageIndex { 4 \! Calculus video tutorial provides a basic introduction into concavity and inflection points of inflection and concavity quick and with... Dimplekumar Chalishajar of VMI and Brian Heinold of Mount Saint Mary 's University maxima or minima value of \ f! This fails we can not conclude concavity changes sign from plus to or... 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