{"id":11876,"date":"2018-02-01t11:19:51","date_gmt":"2018-02-01t19:19:51","guid":{"rendered":"\/\/www.catharsisit.com\/hs\/?p=11876"},"modified":"2018-02-01t11:19:51","modified_gmt":"2018-02-01t19:19:51","slug":"ap-calculus-review-estimating-derivatives-graphs","status":"publish","type":"post","link":"\/\/www.catharsisit.com\/hs\/ap\/ap-calculus-review-estimating-derivatives-graphs\/","title":{"rendered":"ap calculus review: estimating derivatives from graphs"},"content":{"rendered":"
so you may have memorized all of the derivative rules. you might be able to derive f<\/em> '(x) from f<\/em>(x<\/em>) no matter how complicated the function is. but how are you at estimating derivatives directly from the graph?<\/p>\n as we’ll see in this review article, it’s all about slope<\/strong><\/em>!<\/p>\n let’s begin with the fundamental connection between derivatives and graphs of functions.<\/p>\n the derivative value f<\/em> '(a) equals the slope of the tangent line to the graph of y<\/em> = f<\/em>(x<\/em>) at x<\/em> = a<\/em>.<\/strong><\/p>\n i recommend brushing up on the idea of tangent lines first. here are a few resources that might help.<\/p>\n use the information in the graph of f<\/em>(x<\/em>) below to estimate the value of f<\/em> '(1).<\/p>\n remember, derivative values are slopes! so f<\/em> '(1) is equal to the slope of the tangent line attached to the graph at x<\/em> = 1. <\/p>\n all it takes is two points on a line to determine slope. one point is easy to spot because it’s also on the graph of f<\/em> itself: (1, 1). next we look along the tangent line until we find another point whose coordinates are easy to estimate. try to find a point that crosses an “intersection,” because then it will have integer coordinates. for example, (2, 3), or (3, 5), or (0, -1), etc.<\/p>\n i’m going to pick (3, 5) as my second point. however, if you pick any other point, as long as it’s on the tangent line, then your answer should be equal (or very close) to mine.<\/p>\n next, use the slope formula (rise over run<\/em>) to compute the slope of the tangent line.<\/p>\n therefore f<\/em> '(1) = 2.<\/p>\n ok, so the first example may have been pretty easy. how hard can it get?<\/p>\n sometimes we have to estimate all<\/em> of the derivative values! in other words, given the graph of a function f<\/em>(x<\/em>), it should be possible to sketch a graph of f<\/em> '(x<\/em>).<\/p>\n there are three things to keep in mind for differentiable functions.<\/p>\n sketch the graph of the derivative of the function whose graph is shown below.<\/p>\n first identify the two turnaround points: at x<\/em> = -2 and 0. this means that f<\/em> '(-2) = f<\/em> '(0) = 0.<\/p>\n then, identify the intervals on which the graph increases and decreases. when f<\/em> is increasing, we have f<\/em> ' > 0. when f<\/em> is decreasing, we have f<\/em> ' < 0. <\/p>\n the graph below shows the original in black and a sketch of its derivative in blue.<\/p>\n notice how the blue curve fits the description of f<\/em> '.<\/p>\n the methods for estimating derivatives so far have ignored an essential issue. what happens when the function fails to have a derivative value at a given point?<\/p>\n any point x<\/em> = a<\/em> at which f<\/em> '(a<\/em>) does not exist is called a point of non-differentiability.<\/strong><\/p>\n if a<\/em> is such a point, then there will either be a hole or break in the graph of f<\/em> ' at x<\/em> = a<\/em>.<\/p>\n three things could cause such behavior.<\/p>\n let’s explore the three situations in the following example.<\/p>\n sketch the graph of the derivative of the following function.<\/p>\n there is a lot going on in this graph!<\/p>\n aside from these important landmarks, there’s also one turnaround point, at x<\/em> = 0. let’s analyze what happens in the intervals between the special points.<\/p>\n but what exactly happens near x<\/em> = -5, 5, and 8?<\/p>\n at x<\/em> = -5, the original graph follows a vertical asymptote. by definition, the function values are approaching ∞ or -∞ the closer x<\/em> gets to -5. as a result, the function gets infinitely steep as x<\/em> → -5. infinite steepness means infinite slope values, so f<\/em> ' must also have a vertical asymptote at x<\/em> = -5.<\/p>\n next, the corner point at x<\/em> = 5 represents a very sudden change in direction. rather than turning around smoothly, the function instantly changes course. that means that there will be a jump in the derivative value when crossing x<\/em> = 5.<\/p>\nthe derivative measures slope<\/h2>\n
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example — estimating derivatives using tangent lines<\/h3>\n
solution<\/h4>\n
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increasing, decreasing, and turning around<\/h3>\n
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example — estimating the graph of a derivative<\/h3>\n
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solution<\/h4>\n
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non-differentiable points<\/h2>\n
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example — estimating derivatives with non-differentiable points<\/h3>\n
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solution<\/h4>\n
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