5.2: The Definite Integral

In the preceding section we defined the area under a curve in terms of Riemann sums:

However, this definition came with restrictions. We required to be continuous and nonnegative. Unfortunately, real-world problems don’t always meet these restrictions. In this section, we look at how to apply the concept of the area under the curve to a broader set of functions through the use of the definite integral.

Definition and Notation

The definite integral generalizes the concept of the area under a curve. We lift the requirements that be continuous and nonnegative, and define the definite integral as follows. Definition

Definition: definite integral

If is a function defined on an interval the definite integral of f from a to b is given by

provided the limit exists. If this limit exists, the function is said to be integrable on [a,b], or is an integrable function.

The integral symbol in the previous definition should look familiar. We have seen similar notation in the chapter on Applications of Derivatives, where we used the indefinite integral symbol (without the a and b above and below) to represent an antiderivative. Although the notation for indefinite integrals may look similar to the notation for a definite integral, they are not the same. A definite integral is a number. An indefinite integral is a family of functions. Later in this chapter we examine how these concepts are related. However, close attention should always be paid to notation so we know whether we’re working with a definite integral or an indefinite integral.

Integral notation goes back to the late seventeenth century and is one of the contributions of Gottfried Wilhelm Leibniz, who is often considered to be the codiscoverer of calculus, along with Isaac Newton. The integration symbol ∫ is an elongated S, suggesting sigma or summation. On a definite integral, above and below the summation symbol are the boundaries of the interval, The numbers a and b are x-values and are called the limits of integration; specifically, a is the lower limit and b is the upper limit. To clarify, we are using the word limit in two different ways in the context of the definite integral. First, we talk about the limit of a sum as Second, the boundaries of the region are called the limits of integration.

We call the function the integrand, and the dx indicates that is a function with respect to x, called the variable of integration. Note that, like the index in a sum, the variable of integration is a dummy variable, and has no impact on the computation of the integral. We could use any variable we like as the variable of integration:

Previously, we discussed the fact that if is continuous on then the limit exists and is unique. This leads to the following theorem, which we state without proof.

Continuous Functions Are Integrable

If is continuous on , then f is integrable on

Functions that are not continuous on may still be integrable, depending on the nature of the discontinuities. For example, functions with a finite number of jump discontinuities on a closed interval are integrable.

It is also worth noting here that we have retained the use of a regular partition in the Riemann sums. This restriction is not strictly necessary. Any partition can be used to form a Riemann sum. However, if a nonregular partition is used to define the definite integral, it is not sufficient to take the limit as the number of subintervals goes to infinity. Instead, we must take the limit as the width of the largest subinterval goes to zero. This introduces a little more complex notation in our limits and makes the calculations more difficult without really gaining much additional insight, so we stick with regular partitions for the Riemann sums.

Example : Evaluating an Integral Using the Definition

Note to Calc I students: this example is more detailed than the exercises you need to know how to do.

Use the definition of the definite integral to evaluate Use a right-endpoint approximation to generate the Riemann sum.

Solution

We first want to set up a Riemann sum. Based on the limits of integration, we have and . For , let be a regular partition of Then

Since we are using a right-endpoint approximation to generate Riemann sums, for each i, we need to calculate the function value at the right endpoint of the interval The right endpoint of the interval is , and since P is a regular partition,

Thus, the function value at the right endpoint of the interval is

Then the Riemann sum takes the form

.

Using the summation formula for , we have

.

Now, to calculate the definite integral, we need to take the limit as . We get

.

Exercise

Use the definition of the definite integral to evaluate .

Use a right-endpoint approximation to generate the Riemann sum.

Hint

Use the solving strategy from Example .

Answer

Evaluating Definite Integrals

Evaluating definite integrals this way can be quite tedious because of the complexity of the calculations. Later in this chapter we develop techniques for evaluating definite integrals without taking limits of Riemann sums. However, for now, we can rely on the fact that definite integrals represent the area under the curve, and we can evaluate definite integrals by using geometric formulas to calculate that area. We do this to confirm that definite integrals do, indeed, represent areas, so we can then discuss what to do in the case of a curve of a function dropping below the x-axis.

Example : Using Geometric Formulas to Calculate Definite Integrals

Use the formula for the area of a circle to evaluate .

Solution

The function describes a semicircle with radius 3. To find

we want to find the area under the curve over the interval The formula for the area of a circle is . The area of a semicircle is just one-half the area of a circle, or . The shaded area in Figure covers one-half of the semicircle, or . Thus,

Figure : The value of the integral of the function over the interval is the area of the shaded region.

Exercise

Use the formula for the area of a trapezoid to evaluate .

Hint

Graph the function and calculate the area under the function on the interval

Answer

18 square units

Area and the Definite Integral

When we defined the definite integral, we lifted the requirement that be nonnegative. But how do we interpret “the area under the curve” when is negative?

Net Signed Area

Let us return to the Riemann sum. Consider, for example, the function (shown in Figure) on the interval . Use and choose {} as the left endpoint of each interval. Construct a rectangle on each subinterval of height and width . When is positive, the product represents the area of the rectangle, as before. When is negative, however, the product represents the negative of the area of the rectangle. The Riemann sum then becomes

(Area of rectangles above thex-axis)−(Area of rectangles below thex-axis)

Figure : For a function that is partly negative, the Riemann sum is the area of the rectangles above the x-axis less the area of the rectangles below the x-axis.

Taking the limit as the Riemann sum approaches the area between the curve above the x-axis and the x-axis, less the area between the curve below the x-axis and the x-axis, as shown in Figure. Then,

The quantity is called the net signed area.

Figure : In the limit, the definite integral equals area A1 less area A2, or the net signed area.

Notice that net signed area can be positive, negative, or zero. If the area above the x-axis is larger, the net signed area is positive. If the area below the x-axis is larger, the net signed area is negative. If the areas above and below the x-axis are equal, the net signed area is zero.

Example : Finding the Net Signed Area

Find the net signed area between the curve of the function and the x-axis over the interval

Solution

The function produces a straight line that forms two triangles: one from to and the other from to (Figure). Using the geometric formula for the area of a triangle, , the area of triangle A1, above the axis, is

,

where 3 is the base and is the height. The area of triangle A2, below the axis, is

where 3 is the base and 6 is the height. Thus, the net area is

Figure : The area above the curve and below the x-axis equals the area below the curve and above the x-axis.

Analysis

If A1 is the area above the x-axis and A2 is the area below the x-axis, then the net area is . Since the areas of the two triangles are equal, the net area is zero.

Exercise

Find the net signed area of over the interval , illustrated in the following image.

Hint

Use the solving method described in Example .

Answer

6

Total Area

One application of the definite integral is finding displacement when given a velocity function. If represents the velocity of an object as a function of time, then the area under the curve tells us how far the object is from its original position. This is a very important application of the definite integral, and we examine it in more detail later in the chapter. For now, we’re just going to look at some basics to get a feel for how this works by studying constant velocities.

When velocity is a constant, the area under the curve is just velocity times time. This idea is already very familiar. If a car travels away from its starting position in a straight line at a speed of 75 mph for 2 hours, then it is 150 mi away from its original position (Figure). Using integral notation, we have

.

Figure : The area under the curve tells us how far the car is from its starting point at a given time.

In the context of displacement, net signed area allows us to take direction into account. If a car travels straight north at a speed of 60 mph for 2 hours, it is 120 mi north of its starting position. If the car then turns around and travels south at a speed of 40 mph for 3 hours, it will be back at it starting position (Figure). Again, using integral notation, we have

In this case the displacement is zero.

Figure : The area above the axis and the area below the axis are equal, so the net signed area is zero.

Suppose we want to know how far the car travels overall, regardless of direction. In this case, we want to know the area between the curve and the x-axis, regardless of whether that area is above or below the axis. This is called the total area.

Graphically, it is easiest to think of calculating total area by adding the areas above the axis and the areas below the axis (rather than subtracting the areas below the axis, as we did with net signed area). To accomplish this mathematically, we use the absolute value function. Thus, the total distance traveled by the car is

Bringing these ideas together formally, we state the following definitions.

Definition: net signed area

Let be an integrable function defined on an interval . Let represent the area between and the x-axis that lies above the axis and let represent the area between and the x-axis that lies below the axis. Then, the net signed area between and the x-axis is given by

The total area between and the x-axis is given by

Example : Finding the Total Area

Find the total area between and the x-axis over the interval

Solution

Calculate the x-intercept as (set solve for x). To find the total area, take the area below the x-axis over the subinterval and add it to the area above the x-axis on the subinterval (Figure).

Figure : The total area between the line and the x-axis over is plus .

We have

Then, using the formula for the area of a triangle, we obtain

.

The total area, then, is

.

Exercise

Find the total area between the function and the x-axis over the interval

Hint

Review the solving strategy in Example .

Answer

Properties of the Definite Integral

The properties of indefinite integrals apply to definite integrals as well. Definite integrals also have properties that relate to the limits of integration. These properties, along with the rules of integration that we examine later in this chapter, help us manipulate expressions to evaluate definite integrals.

Rule: Properties of the Definite Integral

1.

If the limits of integration are the same, the integral is just a line and contains no area.

2.

If the limits are reversed, then place a negative sign in front of the integral.

3.

The integral of a sum is the sum of the integrals.

4.

The integral of a difference is the difference of the integrals

5.

for constant c. The integral of the product of a constant and a function is equal to the constant multiplied by the integral of the function.

6.

Although this formula normally applies when c is between a and b, the formula holds for all values of , , and , provided is integrable on the largest interval.

Example : Using the Properties of the Definite Integral

Use the properties of the definite integral to express the definite integral of over the interval as the sum of three definite integrals.

Solution

Using integral notation, we have We apply properties 3. and 5. to get

Exercise

Use the properties of the definite integral to express the definite integral of over the interval as the sum of four definite integrals.

Hint

Use the solving strategy from Example and the properties of definite integrals.

Answer

Example : Using the Properties of the Definite Integral

If it is known that and , find the value of .

Solution

By property 6.,

Thus,

.

Exercise

If it is known that and , find the value of

Hint

Use the solving strategy from Example and the rule on properties of definite integrals.

Answer

Comparison Properties of Integrals

A picture can sometimes tell us more about a function than the results of computations. Comparing functions by their graphs as well as by their algebraic expressions can often give new insight into the process of integration. Intuitively, we might say that if a function is above another function , then the area between and the x-axis is greater than the area between and the x-axis. This is true depending on the interval over which the comparison is made. The properties of definite integrals are valid whether , or . The following properties, however, concern only the case , and are used when we want to compare the sizes of integrals.

Comparison Theorem

i. If for , then

ii. If for , then

iii. If m and M are constants such that for , then

Example : Comparing Two Functions over a Given Interval

Compare and over the interval .

Solution

Graphing these functions is necessary to understand how they compare over the interval Initially, when graphed on a graphing calculator, appears to be above everywhere. However, on the interval , the graphs appear to be on top of each other. We need to zoom in to see that, on the interval is above . The two functions intersect at and (Figure).

Figure : (a) The function appears above the function except over the interval (b) Viewing the same graph with a greater zoom shows this more clearly.

We can see from the graph that over the interval . Comparing the integrals over the specified interval we also see that (Figure). The thin, red-shaded area shows just how much difference there is between these two integrals over the interval

Figure : (a) The graph shows that over the interval where equality holds only at the endpoints of the interval. (b) Viewing the same graph with a greater zoom shows this more clearly.

Key Concepts

• The definite integral can be used to calculate net signed area, which is the area above the x-axis less the area below the x-axis. Net signed area can be positive, negative, or zero.
• The component parts of the definite integral are the integrand, the variable of integration, and the limits of integration.
• Continuous functions on a closed interval are integrable. Functions that are not continuous may still be integrable, depending on the nature of the discontinuities.
• The properties of definite integrals can be used to evaluate integrals.
• The area under the curve of many functions can be calculated using geometric formulas.

Key Equations

• Definite Integral

• Properties of the Definite Integral

for constant c

Glossary

definite integral

a primary operation of calculus; the area between the curve and the x-axis over a given interval is a definite integral

integrable function

a function is integrable if the limit defining the integral exists; in other words, if the limit of the Riemann sums as n goes to infinity exists

integrand

the function to the right of the integration symbol; the integrand includes the function being integrated

limits of integration

these values appear near the top and bottom of the integral sign and define the interval over which the function should be integrated

net signed area

the area between a function and the x-axis such that the area below the x-axis is subtracted from the area above the x-axis; the result is the same as the definite integral of the function

total area

total area between a function and the x-axis is calculated by adding the area above the x-axis and the area below the x-axis; the result is the same as the definite integral of the absolute value of the function

variable of integration

indicates which variable you are integrating with respect to; if it is x, then the function in the integrand is followed by dx

Contributors

• Gilbert Strang (MIT) and Edwin “Jed” Herman (Harvey Mudd) with many contributing authors. This content by OpenStax is licensed with a CC-BY-SA-NC 4.0 license. Download for free at http://cnx.org.