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Graphing the Derivative, Applications Section 3.1b

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Remember, that in graphical terms, the derivative of a function at a given point can be thought of as the slope of the curve at that point… Therefore, we can get a good idea of what the graph of looks like by estimating the slopes at various points along the graph of … The key idea: The slopes of a function are the y-values of its derivative graph!!!

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Graph the derivative of the function f whose graph is shown below. Discuss the behavior of f in terms of the signs and values of f. PointEstimated Slope A 4 B 1 C 0 D –1 E F 0 Note: We do not have a formula for either the function or its derivative, but the graphs are still very revealing…

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Graph the derivative of the function f whose graph is shown below. Discuss the behavior of f in terms of the signs and values of f. We notice that f is decreasing where f is negative and increasing where f is positive. Where f is zero, the graph of f has a horizontal tangent, changing from inc. to dec. (point C) or from dec. to inc. (point F).

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Suppose that the function below represents the depth y (in inches) of water in a ditch alongside a road as a function of time x (in days). 1. What does the graph on the right represent? What units would you use along the y -axis? The graph represents the rate of change of the depth of the water with respect to time. The derivative dy/dx would be measured in inches per day.

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Suppose that the function below represents the depth y (in inches) of water in a ditch alongside a road as a function of time x (in days). 2. Describe what happened to the water in the ditch over the course of the 7-day period. Water is 1 in deep at the start of day 1 and rising rapidly, cont. to rise until end of day 2, where it’s max. depth is 5 in. Then the water level goes down until it reaches a depth of 1 in at the end of day 6. During day 7 the water rises to about 2 in.

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Suppose that the function below represents the depth y (in inches) of water in a ditch alongside a road as a function of time x (in days). 3. Can you describe the weather during the 7 days? When was it the wettest? When was it the driest? The weather appears to have been wettest at the beginning of day 1, and driest at the end of day 4.

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Suppose that the function below represents the depth y (in inches) of water in a ditch alongside a road as a function of time x (in days). 4. How does the graph of the derivative help in finding when the weather was wettest or driest? The highest point on the graph of the derivative shows where the water is rising the fastest, while the lowest point on this graph shows where the water is declining the fastest.

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Sketch the graph of a function f that has the following properties: i. ii. the graph of is shown below; iii. is continuous for all x.

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Find the lines that are (a) tangent and (b) normal to the given curve at the given point. Tangent: m = –10, point (–2,15) Normal: m = 1/10, point (–2,15)

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Try #14, 15, and 16 on p.102… 14. The function that is its own derivative: How did we arrive at this answer graphically??? (0,1)

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Try #14, 15, and 16 on p.102… 15. (a) The amount of daylight is increasing the fastest when the slope is greatest sometime around April 1. Rate at this time 4 hours 24 days 1 6 = hours/day (b) The rate of change of daylight appears to be zero when the tangent to the graph is horizontal January 1 and July 1. (c) Positive: January 1 through July 1 Negative: July 1 through December 31

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Try #14, 15, and 16 on p.102… 16. The slope of the graph is zero at about x = 1 and x = –2… The derivative graph includes the points (–2,0) and (1,0). The slopes at x = –3 and x = 2 are about 5 and the slope at x = –0.5 is about –2.5… The derivative graph includes the points (–3,5), (2,5), (–0.5,–2.5).

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