2.1 Limits, Rates of Change, and Tangent Lines Devil’s Tower, Wyoming Greg Kelly, Hanford High School, Richland, Washington Photo by Vickie Kelly, 1993

Suppose you drive 200 miles, and it takes you 4 hours. Then your average speed is: If you look at your speedometer during this trip, it might read 65 mph. This is your instantaneous speed.

A rock falls from a high cliff. The position of the rock is given by: After 2 seconds: average speed: What is the instantaneous speed at 2 seconds?

for some very small change in t where h = some very small change in t We can use the TI-89 to evaluate this expression for smaller and smaller values of h.

We can see that the velocity approaches 64 ft/sec as h becomes very small. 1 80 0.1 65.6 .01 64.16 .001 64.016 .0001 64.0016 .00001 64.0002 We say that the velocity has a limiting value of 64 as h approaches zero. (Note that h never actually becomes zero.)

The limit as h approaches zero:

Section 2.1, Figure 1 Page 60

The slope of a line is given by: The slope at (1,1) can be approximated by the slope of the secant through (4,16). We could get a better approximation if we move the point closer to (1,1). ie: (3,9) Even better would be the point (2,4).

The slope of a line is given by: If we got really close to (1,1), say (1.1,1.21), the approximation would get better still How far can we go?

Section 2.1, Figure 4 Page 62

Section 2.1, Figure 5 Page 63

slope slope at The slope of the curve at the point is:

The slope of the curve at the point is: is called the difference quotient of f at a. If you are asked to find the slope using the definition or using the difference quotient, this is the technique you will use.

The slope of a curve at a point is the same as the slope of the tangent line at that point. In the previous example, the tangent line could be found using . If you want the normal line, use the negative reciprocal of the slope. (in this case, ) (The normal line is perpendicular.)

Review:  velocity = slope These are often mixed up by Calculus students! average slope: slope at a point: average velocity: So are these! instantaneous velocity: If is the position function: velocity = slope 