Lecture 14 Notes: Definite Integrals

18.01 Calculus Jason Starr Fall 2005 Lecture 14. October 14, 2005 Homework. Problem Set 4 Part II: Problem 2. Practice Problems. Course Reader: 3B­1, 3B­3, 3B­4, 3B­5. 1. The problem of areas. The ancient Greeks computed the areas of triangles, quadrilaterals, and many other polygons. Their basic method was dissection: dissecting a polygonal region exactly into smaller regions, usually triangles, having known areas. The area of the large region is the sum of the areas of the small regions. But the ancient Greeks also knew the area of a circle, which cannot be dissected exactly into finitely many polygonal regions. Their method was exhaustion: finding polygonal regions approximately equal to the original region, and computing the limit of the areas of the polygons as the approximation improves. Example. A regular N ­sided polygon inscribed in a circle of radius r has apothem length a = r cos(π/N ) and chord length b = 2r sin(π/N ). Thus the area of the polygon is, A=N ab N sin(2π/N ) = N r2 sin(π/N ) cos(π/N ) = r2 sin(2π/N ) = πr2 . 2 2 2π/N As N increases, 2π/N decreases to 0. Because limt→0 sin(t)/t equals 1, as N approaches infinity, the area of the polygon approaches, lim πr2 N →∞ sin(2π/N ) = πr2 . 2π/N A more sophisticated version of the method of exhaustion gives the Riemann integral. Here is the basic problem. Problem (Area). Find the signed area between the graph of y = f (x) and the x­axis over the interval a ≤ x ≤ b. 18.01 Calculus Jason Starr Fall 2005 For a region above the x­axis, the signed area is simply the area. For a region below the x­axis, the signed area is the negative of the area. For a region partly above the x­axis and partly below the x­axis, the signed area is the sum of the signed area of the region above the x­axis and the signed area of the region below the x­axis. 2. Partitions. A partition of an interval [a, b] is a finite decomposition of the interval as a union of non­overlapping subintervals, [a, b] = [x0 , x1 ] ∪ [x1 , x2 ] ∪ · · · ∪ [xn−2 , xn−2 ] ∪ [xn−1 , xn ]. Since an interval is determined by its right and left endpoints, to specify a partition of [a, b], it is equivalent to give an ordered sequence of increasing numbers, a = x0 < x1 < x2 < · · · < xn−2 < xn−1 < xn = b. The k th subinterval of the partition is the interval [xk−1 , xk ], having length, Δxk = xk − xk−1 . A partition is fine if the subintervals are small, and coarse if the subintervals are large. It may seem the number of intervals n is a good measure of fineness: since the subintervals of a fine partition are small, the number n of subintervals must be large. However, a partition into many subintervals may include a few subintervals that are quite large. For instance, the partition [0, 1] = [0, 1/2n]∪[1/2n, 2/2n]∪[2/2n, 3/2n] ∪· · ·∪[(n−2)/2n, (n−1)/2n] ∪ [n−1/2n, n/2n]∪[1/2, 1], has n very small intervals of length 1/2n, but has one interval, [1/2, 1], of size 1/2. The number 1/2 may not seem large, but as n increases, it is quite large compared to 1/2n. Because of such examples, a better measure of fineness is mesh size: The mesh size of a partition is the maximal length of any subinterval in the partition, mesh = max Δxk |k = 1, . . . , n. 3. Riemann sums. Let f (x) be a function defined on an interval a ≤ x ≤ b. Given a partition a = x0 < · · · < xn = b of [a, b], and given a choice, for every k = 1, . . . , n, of element x∗k in the k th subinterval, xk−1 ≤ x∗k ≤ xk , the curvilinear region bounded by y = f (x) and the x­axis is approximated by a union of n vertical strips. The k th vertical strip lies above or below the interval on the x­axis, xk−1 ≤ x ≤ xk , and has height yk∗ = f (x∗k ). The width of the vertical strip is Δxk , thus the signed area is, ΔAk = yk∗ Δxk . The total area of the union of vertical strips is simply the sum of the areas of individual vertical strips, n � A= yk∗ Δxk . k=1 18.01 Calculus Jason Starr Fall 2005 The sum above is a Riemann sum. It is an approximation of the signed area of the curvilinear region. There are many choices of partition. And for each partition, there are many choices for the numbers x∗k . However, there are some special choices. On the k th interval, the smallest value f (x) takes on is denoted by, yk,min = min{f (x)|xk−1 ≤ x ≤ xk+1 }. Similarly, the largest value f (x) takes on is denote by, yk,max = max{f (x)|xk−1 ≤ x ≤ xk+1 }. For every choice of x∗k in the k th interval, yk∗ is trapped between these two values, yk,min ≤ yk∗ ≤ yk,max . Denoting, ΔAk,min = yk,min Δxk , ΔAk,max = yk,max Δxk , the area ΔAk is trapped between these two values, ΔAk,min ≤ ΔAk ≤ ΔAk,max . Denoting the sums of the areas by, � � Amin = �nk=1 ΔAk,min = �nk=1 yk,min Δxk , Amax = nk=1 ΔAk,min = nk=1 yk,min Δxk , the Riemann sum A is trapped between the two values, Amin ≤ A ≤ Amax . Thus, if Amin and Amax are close to each other, the value of A does not depend very much on the choices of the numbers x∗k . 4. The Riemann integral. The method of the Riemann integral is to compute both Amin and Amax for a sequence of partitions whose mesh sizes approach 0. The mesh size measures the fineness of the partition, thus also the fit of the union of vertical strips to the curvilinear region. If the two limits, lim Amin , lim Amax , mesh→0 mesh→0 are defined and equal, it is said the Riemann integral exists, and the common limit is called the Riemann integral, � b f (x)dx = lim Amin = lim Amax . a mesh→0 mesh→0 Also, f (x) is said to be Riemann integrable on the interval [a, b]. Another name for the Riemann integral is the definite integral. 18.01 Calculus Jason Starr Fall 2005 Example. Consider the function f (x) = x on the interval 0 ≤ x ≤ L, for some positive number L. Form the partition with n subintervals of equal length, x0 = 0 = 0L/n, x1 = 1L/n, x2 = 2L/n, . . . , xk = kL/n, . . . xn = nL/n = L. Every interval has length Δxk = L/n. So the mesh size is L/n. The minimum value of f (x) on the interval xk−1 ≤ x ≤ xk is yk,min = xk−1 = (k − 1)L/n. The maximum value is yk,max = xk = kL/n. Thus, n n n � � (k − 1)L L L2 � (k − 1), Amin = yk,min Δxk = = 2 n n n k=1 k=1 k=1 and, Amax = n � yk,max Δxk = k=1 n � kL L k=1 n n = n L2 � k. n2 k=1 To evaluate these sums, use the well­known formula, n � k= k=1 n(n + 1) . 2 This also gives, n � (k − 1) = k=1 n−1 � l=0 l= n−1 � l=1 l= (n − 1)n , 2 by making the substitution l = k − 1. Substituting the formula gives, Amin = L2 n(n − 1) L2 1 = (1 − ), n2 2 2 n and, Amin L2 n(n + 1) L2 1 = 2 = (1 + ). n 2 2 n Therefore, lim Amin n→∞ L2 1 L2 L2 = lim(1 − ) = (1 − 0) = . 2 n→0 n 2 2 Similarly, L2 1 L2 L2 lim(1 + ) = (1 + 0) = . n→∞ 2 n→0 n 2 2 Since the two limits are equal, f (x) = x is Riemann integrable on the interval [0, L], and, � L xdx = L2 /2. lim Amax = 0 This agrees with the familiar result from high­school geometry: the area of a triangle equals one half of the base times the height, since both the base and height of this triangle are L. 18.01 Calculus Jason Starr Fall 2005 5. Rules for Riemann integrals. There are several rules for Riemann integrals, summarized below. �b �b �b (f (x) + g(x))dx = a f (x)dx + a g(x)dx, a� �b b (r f (x))dx = r a f (x)dx, a b c c f (x)dx + b f (x)dx = f (x)dx. a a