4.4 the Fundamental Theorem of Calculus 277
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4.4 The Fundamental Theorem of Calculus 277 4.4 The Fundamental Theorem of Calculus Evaluate a definite integral using the Fundamental Theorem of Calculus. Understand and use the Mean Value Theorem for Integrals. Find the average value of a function over a closed interval. Understand and use the Second Fundamental Theorem of Calculus. Understand and use the Net Change Theorem. The Fundamental Theorem of Calculus You have now been introduced to the two major branches of calculus: differential calculus (introduced with the tangent line problem) and integral calculus (introduced with the area problem). So far, these two problems might seem unrelated—but there is a very close connection. The connection was discovered independently by Isaac Newton and Gottfried Leibniz and is stated in the Fundamental Theorem of Calculus. Informally, the theorem states that differentiation and (definite) integration are inverse operations, in the same sense that division and multiplication are inverse operations. To see how Newton and Leibniz might have anticipated this relationship, consider the approximations shown in Figure 4.27. The slope of the tangent line was defined using the quotient ⌬y͞⌬x (the slope of the secant line). Similarly, the area of a region under a curve was defined using the product ⌬y⌬x (the area of a rectangle). So, at least in the primitive approximation stage, the operations of differentiation and definite integration appear to have an inverse relationship in the same sense that division and multiplication are inverse operations. The Fundamental Theorem of Calculus states that the limit processes (used to define the derivative and definite integral) preserve this inverse relationship. Δx Δx Area of rectangle Tangent Δy Δy Area of Secant line region line under curve Δy Δy Slope = Slope ≈ Δx Δx Area = ΔyΔx Area ≈ ΔyΔx (a) Differentiation (b) Definite integration Differentiation and definite integration have an “inverse” relationship. Figure 4.27 ANTIDIFFERENTIATION AND DEFINITE INTEGRATION Throughout this chapter, you have been using the integral sign to denote an antiderivative (a family of functions) and a definite integral (a number). b Antidifferentiation:͵ f͑x͒ dx Definite integration: ͵ f͑x͒ dx a The use of the same symbol for both operations makes it appear that they are related. In the early work with calculus, however, it was not known that the two operations were related.The symbol ͐ was first applied to the definite integral by Leibniz and was derived from the letter S. (Leibniz calculated area as an infinite sum, thus, the letter S.͒ Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 278 Chapter 4 Integration THEOREM 4.9 The Fundamental Theorem of Calculus If a function f is continuous on the closed interval ͓a, b͔ and F is an antiderivative of f on the interval ͓a, b͔, then b ͵ ͑ ͒ ϭ ͑ ͒ Ϫ ͑ ͒ f x dx F b F a . a Proof The key to the proof is writing the difference F͑b͒ Ϫ F͑a͒ in a convenient form. Let ⌬ be any partition of ͓a, b͔. ϭ . ϭ a x0 < x1 < x2 < < xnϪ1 < xn b By pairwise subtraction and addition of like terms, you can write ͑ ͒ Ϫ ͑ ͒ ϭ ͑ ͒ Ϫ ͑ ͒ ϩ ͑ ͒ Ϫ . Ϫ ͑ ͒ ϩ ͑ ͒ Ϫ ͑ ͒ F b F a F xn F xnϪ1 F xnϪ1 F x1 F x1 F x0 n ϭ ͓ ͑ ͒ Ϫ ͑ ͔͒ ͚ F xi F xiϪ1 . iϭ1 By the Mean Value Theorem, you know that there exists a number ci in the ith subinterval such that F͑x ͒ Ϫ F͑x ͒ Ј͑ ͒ ϭ i iϪ1 F ci Ϫ . xi xiϪ1 Ј͑ ͒ ϭ ͑ ͒ ⌬ ϭ Ϫ Because F ci f ci , you can let xi xi xiϪ1 and obtain n ͑ ͒ Ϫ ͑ ͒ ϭ ͑ ͒ ⌬ F b F a ͚ f ci xi . iϭ1 This important equation tells you that by repeatedly applying the Mean Value Theorem, ͑ ͒ Ϫ ͑ ͒ you can always find a collection of ci’s such that the constant F b F a is a Riemann sum of f on ͓a, b͔ for any partition. Theorem 4.4 guarantees that the limit of Riemann sums over the partition with ʈ⌬ʈ → 0 exists. So, taking the limit ͑as ʈ⌬ʈ → 0͒ produces b ͑ ͒ Ϫ ͑ ͒ ϭ ͵ ͑ ͒ F b F a f x dx. a See LarsonCalculus.com for Bruce Edwards’s video of this proof. GUIDELINES FOR USING THE FUNDAMENTAL THEOREM OF CALCULUS 1. Provided you can find an antiderivative of f, you now have a way to evaluate a definite integral without having to use the limit of a sum. 2. When applying the Fundamental Theorem of Calculus, the notation shown below is convenient. b b ͵ f͑x͒ dx ϭ F͑x͒΅ ϭ F͑b͒ Ϫ F͑a͒ a a ͐3 3 For instance, to evaluate 1 x dx, you can write 3 x4 3 34 14 81 1 ͵ x3 dx ϭ ΅ ϭ Ϫ ϭ Ϫ ϭ 20. 1 4 1 4 4 4 4 3. It is not necessary to include a constant of integration C in the antiderivative. b b ͵ f͑x͒ dx ϭ ΄F͑x͒ ϩ C΅ ϭ ͓F͑b͒ ϩ C͔ Ϫ ͓F͑a͒ ϩ C͔ ϭ F͑b͒ Ϫ F͑a͒ a a Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 4.4 The Fundamental Theorem of Calculus 279 Evaluating a Definite Integral See LarsonCalculus.com for an interactive version of this type of example. Evaluate each definite integral. 2 4 ͞4 ͵ ͑ 2 Ϫ ͒ ͵ Ί ͵ 2 a. x 3 dx b. 3 x dx c. sec x dx 1 1 0 Solution 2 x3 2 8 1 2 a. ͵ ͑x2 Ϫ 3͒ dx ϭ ΄ Ϫ 3x΅ ϭ Ϫ 6 Ϫ Ϫ 3 ϭϪ 1 3 1 3 3 3 4 4 x3͞2 4 Ί ϭ 1͞2 ϭ ϭ ͑ ͒3͞2 Ϫ ͑ ͒3͞2 ϭ b. ͵ 3 x dx 3͵ x dx 3΄ ͞ ΅ 2 4 2 1 14 1 1 3 2 1 ͞4 ͞4 c. ͵ sec2 x dx ϭ tan x΅ ϭ 1 Ϫ 0 ϭ 1 0 0 y y = ⏐2x − 1⏐ A Definite Integral Involving Absolute Value 2 3 Evaluate ͵ Խ2x Ϫ 1Խ dx. 0 2 Solution Using Figure 4.28 and the definition of absolute value, you can rewrite the integrand as shown. Ϫ͑2x Ϫ 1͒, x < 1 1 Խ2x Ϫ 1Խ ϭ Ά 2 Ϫ Ն 1 2x 1, x 2 x From this, you can rewrite the integral in two parts. − 112 2 1͞2 2 y = −(2x − 1) y = 2x − 1 ͵ Խ2x Ϫ 1Խ dx ϭ ͵ Ϫ͑2x Ϫ 1͒ dx ϩ ͵ ͑2x Ϫ 1͒ dx 0 0 1͞2 ͞ The definite integral of y on ͓0, 2͔ is 5. 1 2 2 2 ϭ ΄Ϫx2 ϩ x΅ ϩ ΄x2 Ϫ x΅ Figure 4.28 0 1͞2 1 1 1 1 ϭ Ϫ ϩ Ϫ ͑0 ϩ 0͒ ϩ ͑4 Ϫ 2͒ Ϫ Ϫ 4 2 4 2 5 ϭ 2 y Using the Fundamental Theorem to Find Area y = 2x2 − 3x + 2 Find the area of the region bounded by the graph of 4 y ϭ 2x2 Ϫ 3x ϩ 2 3 the x-axis, and the vertical lines x ϭ 0 and x ϭ 2, as shown in Figure 4.29. 2 Solution Note that y > 0 on the interval ͓0, 2͔. 2 ϭ ͵ ͑ 2 Ϫ ϩ ͒ Integrate between x ϭ 0 and x ϭ 2. 1 Area 2x 3x 2 dx 0 2x3 3x2 2 x ϭ ΄ Ϫ ϩ 2x΅ Find antiderivative. 1234 3 2 0 16 The area of the region bounded by the ϭ Ϫ 6 ϩ 4 Ϫ ͑0 Ϫ 0 ϩ 0͒ Apply Fundamental Theorem. graph of y, the x-axis,x ϭ 0, and 3 x ϭ 2 is 10. 10 3 ϭ Simplify. Figure 4.29 3 Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 280 Chapter 4 Integration The Mean Value Theorem for Integrals y In Section 4.2, you saw that the area of a region under a curve is greater than the area of an inscribed rectangle and less than the area of a circumscribed rectangle. The Mean Value Theorem for Integrals states that somewhere “between” the inscribed and circum- scribed rectangles, there is a rectangle whose area is precisely equal to the area of the region under the curve, as shown in Figure 4.30. f f(c) x THEOREM 4.10 Mean Value Theorem for Integrals acb If f is continuous on the closed interval ͓a, b͔, then there exists a number c in Mean value rectangle: the closed interval ͓a, b͔ such that b b f͑c͒͑b Ϫ a͒ ϭ ͵ f͑x͒ dx ͵ ͑ ͒ ϭ ͑ ͒͑ Ϫ ͒ a f x dx f c b a .