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The Definite Integral

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The Definite Integral Mathematics Learning Centre Introduction to Integration Part 2: The Definite Integral Mary Barnes c 1999 University of Sydney Contents 1Introduction 1 1.1 Objectives . ................................. 1 2 Finding Areas 2 3 Areas Under Curves 4 3.1 What is the point of all this? ......................... 6 3.2 Note about summation notation ........................ 6 4 The Definition of the Definite Integral 7 4.1 Notes . ..................................... 7 5 The Fundamental Theorem of the Calculus 8 6 Properties of the Definite Integral 12 7 Some Common Misunderstandings 14 7.1 Arbitrary constants . ............................. 14 7.2 Dummy variables . ............................. 14 8 Another Look at Areas 15 9 The Area Between Two Curves 19 10 Other Applications of the Definite Integral 21 11 Solutions to Exercises 23 Mathematics Learning Centre, University of Sydney 1 1Introduction This unit deals with the definite integral.Itexplains how it is defined, how it is calculated and some of the ways in which it is used. We shall assume that you are already familiar with the process of finding indefinite inte- grals or primitive functions (sometimes called anti-differentiation) and are able to ‘anti- differentiate’ a range of elementary functions. If you are not, you should work through Introduction to Integration Part I: Anti-Differentiation, and make sure you have mastered the ideas in it before you begin work on this unit. 1.1 Objectives By the time you have worked through this unit you should: • Be familiar with the definition of the definite integral as the limit of a sum; • Understand the rule for calculating definite integrals; • Know the statement of the Fundamental Theorem of the Calculus and understand what it means; • Be able to use definite integrals to find areas such as the area between a curve and the x-axis and the area between two curves; • Understand that definite integrals can also be used in other situations where the quantity required can be expressed as the limit of a sum. Mathematics Learning Centre, University of Sydney 2 2 Finding Areas Areas of plane (i.e. flat!) figures are fairly easy to calculate if they are bounded by straight lines. The area of a rectangle is clearly the length times the breadth. The area of a right-angled triangle can be seen to be half the area of a rectangle (see the diagram) and so is half the base Area of rectangle Area of triangle =length × breadth = 1 area of rectangle times the height. 2 1 × = 2 length breadth The areas of other triangles can be found A A by expressing them as the sum or the dif- ference of the areas of right angled trian- gles, and from this it is clear that for any triangle this area is half the base times the height. BDCBCD Area of ABC Area of ABC = area of ABD = area of ABD + area of ACD − area of ACD Using this, we can find the area of any figure bounded by straight lines, by di- viding it up into triangles (as shown). Areas bounded by curved lines are a much more difficult problem, however. In fact, although we all feel we know intuitively what we mean by the area of a curvilinear figure, it is actually quite difficult to define precisely. The area of a figure is quantified by asking ‘how many units of area would be needed to cover it?’ We need to have some unit of area in mind (e.g. one square centimetre or one square millimetre) and imagine trying to cover the figure with little square tiles. We can also imagine cutting these tiles in halves, quarters etc. In this way a rectangle, and hence any figure bounded by straight lines, can be dealt with, but a curvilinear figure can never be covered exactly. We are therefore forced to rely on the notion of limit in order to define areas of curvilinear figures. To do this, we make some simple assumptions which most people will accept as intuitively obvious. These are:- 1. If one figure is a subset of a second figure, then the area of the first will be less than or equal to that of the second. Mathematics Learning Centre, University of Sydney 3 2. If a figure is divided up into non-overlapping pieces, the area of the whole will be the sum of the areas of the pieces. Using these assumptions, we can approximate to curved figures by means of polygons (figures with straight line boundaries), and hence define the area of the curved figure as the limit of the areas of the polygons as they ‘approach’ the curved figure (in some sense yettobemade precise). Mathematics Learning Centre, University of Sydney 4 3 Areas Under Curves Let us suppose that we are given a positive function y f(x) and we want to find the area enclosed between y = f(x) the curve y = f(x), the x-axis and the lines x = a and x = b. (The shaded area in the diagram.) If the graph of y = f(x)isnot a straight line we do not, at the moment, know how to calculate the area Area precisely. abx We can, however, approximate to the area as follows: y First we divide the area up into strips as shown, by y = f(x) dividing the interval from a to b into equal subinter- vals, and drawing vertical lines at these points. absubdivisions x Next we choose the least value of f(x)ineach subin- y terval and construct a rectangle with that as its height (as in the diagram). The sum of the areas of these rectangles is clearly less than the area we are trying to find. This sum is called a lower sum. ablower sum x Then we choose the greatest value of f(x)ineach y subinterval and construct a rectangle with that as its height (as in the diagram opposite). The sum of the areas of these rectangles is clearly greater than the area we are trying to find. This sum is called an upper sum. abupper sum x Thus we have ‘sandwiched’ the area we want to find in between an upper sum and a lower sum. Both the upper sum and the lower sum are easily calculated because they are sums of areas of rectangles. Mathematics Learning Centre, University of Sydney 5 Although we still can’t say precisely what the area under the curve is, we know between what limits it lies. If we now increase the number of strips the area is y divided into, we will get new upper and lower sums, which will be closer to one another in size and so closer to the area which we are trying to find. In fact, the larger the number of strips we take, the smaller will be the difference between the upper and lower sums, and so the better approximation either sum will be to the area under the curve. division into narrower stripsx It can be shown that if f(x)isa‘nice’ function (for example, a continuous function) the difference between the upper and lower sums approaches zero as the number of strips the area is subdivided into approaches infinity. We can thus define the area under the curve to be: the limit of either the upper sum or the lower sum, as the number of subdivi- sions tends to infinity (and the width of each subdivision tends to zero). Thus finding the area under a curve boils down to finding the limit of a sum. Now let us introduce some notation so that we can talk more precisely about these con- cepts. Let us suppose that the interval [a, b]isdivided into y n equal subintervals each of width ∆x. Suppose also y = f(x) that the greatest value of f(x)inthe ith subinterval ∗ is f(xi ) and the least value is f(xi). Then the upper sum can be written as: f(x*) f(x') ∗ ∗ ∗ f(x1)∆x + f(x2)∆x + ... + f(xn)∆x n ∗ or, using summation notation: i=1 f(xi )∆x. ∆∆x x Similarly, the lower sum can be written as: abx f(x1)∆x + f(x2)∆x + ... + f(xn)∆x n or, using summation notation: i=1 f(xi)∆x. With this notation, and letting A stand for the area under the curve y = f(x) from x = a to x = b,wecan express our earlier conclusions in symbolic form. The area lies between the lower sum and the upper sum and can be written as follows: n n ≤ ≤ ∗ f(xi)∆x A f(xi )∆x. i=1 i=1 Mathematics Learning Centre, University of Sydney 6 The area is equal to the limit of the lower sum or the upper sum as the number of subdivisions tends to infinity and can be written as follows: n A = lim f(x )∆x n→∞ i i=1 or n A = lim f(x∗)∆x. n→∞ i i=1 3.1 What is the point of all this? Well, firstly it enables us to define precisely what up till now has only been an impre- cise intuitive concept, namely, the area of a region with curved lines forming part of its boundary. Secondly it indicates how we may calculate approximations to such an area. By taking a fairly large value of n and finding upper or lower sums we get an approximate value for the area. The difference between the upper and lower sums tells us how accurate this approximation is. This, unfortunately, is not a very good or very practical way of approximating to the area under a curve. If you do a course in Numerical Methods you will learn much better ways, such as the Trapezoidal Rule and Simpson’s Rule.
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