As we embark on our study of calculus, we shall see how its development arose from common solutions to practical problems in areas such as engineering physics—like the space travel problem posed in the chapter opener. Two key problems led to the initial formulation of calculus: (1) the tangent problem, or how to determine the slope of a line tangent to a curve at a point; and (2) the area problem, or how to determine the area under a curve.
Rate of change is one of the most critical concepts in calculus. We begin our investigation of rates of change by looking at the graphs of the three lines
and
shown in [link].
As we move from left to right along the graph of
we see that the graph decreases at a constant rate. For every 1 unit we move to the right along the x-axis, the y-coordinate decreases by 2 units. This rate of change is determined by the slope (−2) of the line. Similarly, the slope of 1/2 in the function
tells us that for every change in x of 1 unit there is a corresponding change in y of 1/2 unit. The function
has a slope of zero, indicating that the values of the function remain constant. We see that the slope of each linear function indicates the rate of change of the function.
Compare the graphs of these three functions with the graph of
([link]). The graph of
starts from the left by decreasing rapidly, then begins to decrease more slowly and level off, and then finally begins to increase—slowly at first, followed by an increasing rate of increase as it moves toward the right. Unlike a linear function, no single number represents the rate of change for this function. We quite naturally ask: How do we measure the rate of change of a nonlinear function?
We can approximate the rate of change of a function
at a point
on its graph by taking another point
on the graph of
drawing a line through the two points, and calculating the slope of the resulting line. Such a line is called a secant line. [link] shows a secant line to a function
at a point
We formally define a secant line as follows:
The secant to the function
through the points
and
is the line passing through these points. Its slope is given by
The accuracy of approximating the rate of change of the function with a secant line depends on how close x is to a. As we see in [link], if x is closer to a, the slope of the secant line is a better measure of the rate of change of
at a.
The secant lines themselves approach a line that is called the tangent to the function
at a ([link]). The slope of the tangent line to the graph at a measures the rate of change of the function at a. This value also represents the derivative of the function
at a, or the rate of change of the function at a. This derivative is denoted by
Differential calculus is the field of calculus concerned with the study of derivatives and their applications.
For an interactive demonstration of the slope of a secant line that you can manipulate yourself, visit this applet (Note: this site requires a Java browser plugin): Math Insight.
[link] illustrates how to find slopes of secant lines. These slopes estimate the slope of the tangent line or, equivalently, the rate of change of the function at the point at which the slopes are calculated.
Estimate the slope of the tangent line (rate of change) to
at
by finding slopes of secant lines through
and each of the following points on the graph of
Use the formula for the slope of a secant line from the definition.
The point in part b. is closer to the point
so the slope of 2.5 is closer to the slope of the tangent line. A good estimate for the slope of the tangent would be in the range of 2 to 2.5 ([link]).
We continue our investigation by exploring a related question. Keeping in mind that velocity may be thought of as the rate of change of position, suppose that we have a function,
that gives the position of an object along a coordinate axis at any given time t. Can we use these same ideas to create a reasonable definition of the instantaneous velocity at a given time
We start by approximating the instantaneous velocity with an average velocity. First, recall that the speed of an object traveling at a constant rate is the ratio of the distance traveled to the length of time it has traveled. We define the average velocity of an object over a time period to be the change in its position divided by the length of the time period.
Let
be the position of an object moving along a coordinate axis at time t. The average velocity of the object over a time interval
where
(or
if
is
As t is chosen closer to a, the average velocity becomes closer to the instantaneous velocity. Note that finding the average velocity of a position function over a time interval is essentially the same as finding the slope of a secant line to a function. Furthermore, to find the slope of a tangent line at a point a, we let the x-values approach a in the slope of the secant line. Similarly, to find the instantaneous velocity at time a, we let the t-values approach a in the average velocity. This process of letting x or t approach a in an expression is called taking a limit. Thus, we may define the instantaneous velocity as follows.
For a position function
the instantaneous velocity at a time
is the value that the average velocities approach on intervals of the form
and
as the values of t become closer to a, provided such a value exists.
[link] illustrates this concept of limits and average velocity.
A rock is dropped from a height of 64 ft. It is determined that its height (in feet) above ground t seconds later (for
is given by
Find the average velocity of the rock over each of the given time intervals. Use this information to guess the instantaneous velocity of the rock at time
Substitute the data into the formula for the definition of average velocity.
The instantaneous velocity is somewhere between −15.84 and −16.16 ft/sec. A good guess might be −16 ft/sec.
An object moves along a coordinate axis so that its position at time t is given by
Estimate its instantaneous velocity at time
by computing its average velocity over the time interval
12.006001
Use
We now turn our attention to a classic question from calculus. Many quantities in physics—for example, quantities of work—may be interpreted as the area under a curve. This leads us to ask the question: How can we find the area between the graph of a function and the x-axis over an interval ([link])?
As in the answer to our previous questions on velocity, we first try to approximate the solution. We approximate the area by dividing up the interval
into smaller intervals in the shape of rectangles. The approximation of the area comes from adding up the areas of these rectangles ([link]).
As the widths of the rectangles become smaller (approach zero), the sums of the areas of the rectangles approach the area between the graph of
and the x-axis over the interval
Once again, we find ourselves taking a limit. Limits of this type serve as a basis for the definition of the definite integral. Integral calculus is the study of integrals and their applications.
Estimate the area between the x-axis and the graph of
over the interval
by using the three rectangles shown in [link].
The areas of the three rectangles are 1 unit2, 2 unit2, and 5 unit2. Using these rectangles, our area estimate is 8 unit2.
Estimate the area between the x-axis and the graph of
over the interval
by using the three rectangles shown here:
16 unit2
Use [link] as a guide.
So far, we have studied functions of one variable only. Such functions can be represented visually using graphs in two dimensions; however, there is no good reason to restrict our investigation to two dimensions. Suppose, for example, that instead of determining the velocity of an object moving along a coordinate axis, we want to determine the velocity of a rock fired from a catapult at a given time, or of an airplane moving in three dimensions. We might want to graph real-value functions of two variables or determine volumes of solids of the type shown in [link]. These are only a few of the types of questions that can be asked and answered using multivariable calculus. Informally, multivariable calculus can be characterized as the study of the calculus of functions of two or more variables. However, before exploring these and other ideas, we must first lay a foundation for the study of calculus in one variable by exploring the concept of a limit.
For the following exercises, points
and
are on the graph of the function
[T] Complete the following table with the appropriate values: y-coordinate of Q, the point
and the slope of the secant line passing through points P and Q. Round your answer to eight significant digits.
x | y |
| msec | {: valign=”top”}|———- | 1.1 | a. | e. | i. | {: valign=”top”}| 1.01 | b. | f. | j. | {: valign=”top”}| 1.001 | c. | g. | k. | {: valign=”top”}| 1.0001 | d. | h. | l. | {: valign=”top”}{: summary=”A table with four columns and five rows. The first row has the headings x, y, Q(x,y), and msec, the slope of the secant line. The values under x are 1.1, 1.01, 1.001, and 1.0001. The values under y are a, b, c, and d. The values under Q(x,y) are e, f, g, and h. The values under msec¬ are i, j, k, and l.” .unnumbered}
a. 2.2100000; b. 2.0201000; c. 2.0020010; d. 2.0002000; e. (1.1000000, 2.2100000); f. (1.0100000, 2.0201000); g. (1.0010000, 2.0020010); h. (1.0001000, 2.0002000); i. 2.1000000; j. 2.0100000; k. 2.0010000; l. 2.0001000
Use the values in the right column of the table in the preceding exercise to guess the value of the slope of the line tangent to f at
Use the value in the preceding exercise to find the equation of the tangent line at point P. Graph
and the tangent line.
For the following exercises, points
and
are on the graph of the function
[T] Complete the following table with the appropriate values: y-coordinate of Q, the point
and the slope of the secant line passing through points P and Q. Round your answer to eight significant digits.
x | y |
| msec | {: valign=”top”}|———- | 1.1 | a. | e. | i. | {: valign=”top”}| 1.01 | b. | f. | j. | {: valign=”top”}| 1.001 | c. | g. | k. | {: valign=”top”}| 1.0001 | d. | h. | l. | {: valign=”top”}{: summary=”A table with four columns and five rows. The first row has the headings x, y, Q(x,y), and msec, the slope of the secant line. The values under x are 1.1, 1.01, 1.001, and 1.0001. The values under y are a, b, c, and d. The values under Q(x,y) are e, f, g, and h. The values under msec¬ are i, j, k, and l.” .unnumbered}
Use the values in the right column of the table in the preceding exercise to guess the value of the slope of the tangent line to f at
3
Use the value in the preceding exercise to find the equation of the tangent line at point P. Graph
and the tangent line.
For the following exercises, points
and
are on the graph of the function
[T] Complete the following table with the appropriate values: y-coordinate of Q, the point
and the slope of the secant line passing through points P and Q. Round your answer to eight significant digits.
x | y |
| msec | {: valign=”top”}|———- | 4.1 | a. | e. | i. | {: valign=”top”}| 4.01 | b. | f. | j. | {: valign=”top”}| 4.001 | c. | g. | k. | {: valign=”top”}| 4.0001 | d. | h. | l. | {: valign=”top”}{: summary=”A table with four columns and five rows. The first row has the headings x, y, Q(x,y), and msec, the slope of the secant line. The values under x are 4.1, 4.01, 4.001, and 4.0001. The values under y are a, b, c, and d. The values under Q(x,y) are e, f, g, and h. The values under msec¬ are i, j, k, and l.” .unnumbered}
a. 2.0248457; b. 2.0024984; c. 2.0002500; d. 2.0000250; e. (4.1000000,2.0248457); f. (4.0100000,2.0024984); g. (4.0010000,2.0002500); h. (4.00010000,2.0000250); i. 0.24845673; j. 0.24984395; k. 0.24998438; l. 0.24999844
Use the values in the right column of the table in the preceding exercise to guess the value of the slope of the tangent line to f at
Use the value in the preceding exercise to find the equation of the tangent line at point P.
For the following exercises, points
and
are on the graph of the function
[T] Complete the following table with the appropriate values: y-coordinate of Q, the point
and the slope of the secant line passing through points P and Q. Round your answer to eight significant digits.
x | y |
| msec | {: valign=”top”}|———- | 1.4 | a. | e. | i. | {: valign=”top”}| 1.49 | b. | f. | j. | {: valign=”top”}| 1.499 | c. | g. | k. | {: valign=”top”}| 1.4999 | d. | h. | l. | {: valign=”top”}{: summary=”A table with four columns and five rows. The first row has the headings x, y, Q(phi,y), and msec, the slope of the secant line. The values under x are 1.4, 1.49, 1.499, and 1.4999. The values under y are a, b, c, and d. The values under Q(phi,y) are e, f, g, and h. The values under msec¬ are i, j, k, and l.” .unnumbered}
Use the values in the right column of the table in the preceding exercise to guess the value of the slope of the tangent line to f at
π
Use the value in the preceding exercise to find the equation of the tangent line at point P.
For the following exercises, points
and
are on the graph of the function
[T] Complete the following table with the appropriate values: y-coordinate of Q, the point
and the slope of the secant line passing through points P and Q. Round your answer to eight significant digits.
x | y |
| msec | {: valign=”top”}|———- | −1.05 | a. | e. | i. | {: valign=”top”}| −1.01 | b. | f. | j. | {: valign=”top”}| −1.005 | c. | g. | k. | {: valign=”top”}| −1.001 | d. | h. | l. | {: valign=”top”}{: summary=”A table with four columns and five rows. The first row has the headings x, y, Q(x,y), and msec, the slope of the secant line. The values under x are -1.05, -1.01, -1.005, and -1.001. The values under y are a, b, c, and d. The values under Q(x,y) are e, f, g, and h. The values under msec¬ are i, j, k, and l.” .unnumbered}
a. −0.95238095; b. −0.99009901; c. −0.99502488; d. −0.99900100; e. (−1;.0500000,−0;.95238095); f. (−1;.0100000,−0;.9909901); g. (−1;.0050000,−0;.99502488); h. (1.0010000,−0;.99900100); i. −0.95238095; j. −0.99009901; k. −0.99502488; l. −0.99900100
Use the values in the right column of the table in the preceding exercise to guess the value of the slope of the line tangent to f at
Use the value in the preceding exercise to find the equation of the tangent line at point P.
For the following exercises, the position function of a ball dropped from the top of a 200-meter tall building is given by
where position s is measured in meters and time t is measured in seconds. Round your answer to eight significant digits.
[T] Compute the average velocity of the ball over the given time intervals.
Use the preceding exercise to guess the instantaneous velocity of the ball at
sec.
−49 m/sec (velocity of the ball is 49 m/sec downward)
For the following exercises, consider a stone tossed into the air from ground level with an initial velocity of 15 m/sec. Its height in meters at time t seconds is
[T] Compute the average velocity of the stone over the given time intervals.
Use the preceding exercise to guess the instantaneous velocity of the stone at
sec.
5.2 m/sec
For the following exercises, consider a rocket shot into the air that then returns to Earth. The height of the rocket in meters is given by
where t is measured in seconds.
[T] Compute the average velocity of the rocket over the given time intervals.
Use the preceding exercise to guess the instantaneous velocity of the rocket at
sec.
−9.8 m/sec
For the following exercises, consider an athlete running a 40-m dash. The position of the athlete is given by
where d is the position in meters and t is the time elapsed, measured in seconds.
[T] Compute the average velocity of the runner over the given time intervals.
Use the preceding exercise to guess the instantaneous velocity of the runner at
sec.
6 m/sec
For the following exercises, consider the function
Sketch the graph of f over the interval
and shade the region above the x-axis.
Use the preceding exercise to find the exact value of the area between the x-axis and the graph of f over the interval
using rectangles. For the rectangles, use the square units, and approximate both above and below the lines. Use geometry to find the exact answer.
Under, 1 unit2; over: 4 unit2. The exact area of the two triangles is
For the following exercises, consider the function
(Hint: This is the upper half of a circle of radius 1 positioned at
Sketch the graph of f over the interval
Use the preceding exercise to find the exact area between the x-axis and the graph of f over the interval
using rectangles. For the rectangles, use squares 0.4 by 0.4 units, and approximate both above and below the lines. Use geometry to find the exact answer.
Under, 0.96 unit2; over, 1.92 unit2. The exact area of the semicircle with radius 1 is
unit2.
For the following exercises, consider the function
Sketch the graph of f over the interval
Approximate the area of the region between the x-axis and the graph of f over the interval
Approximately 1.3333333 unit2
(if
or
if
, with a position given by
that is
is the value that the average velocities on intervals of the form
and
approach as the values of t move closer to
provided such a value exists
as x approaches a is the value that
approaches as x approaches a
at a is a line through the point
and another point on the function; the slope of the secant line is given by
is the line that secant lines through
approach as they are taken through points on the function with x-values that approach a; the slope of the tangent line to a graph at a measures the rate of change of the function at a
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