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AP Calculus AB: 9.6.2 An Example of the Trapezoidal Rule

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This section demonstrates how to apply the Trapezoidal Rule to approximate definite integrals, such as estimating ln f(x)=1/x over [1, 3]. It walks through partitioning the interval, calculating function values, and plugging them into the formula, emphasizing how increasing partitions improves accuracy—even when only discrete data points are available.

An Example of the Trapezoidal Rule

• The trapezoidal rule approximates the area A of the region bound by the curve of a continuous function f (x) and the x-axis using N partitions on [a, b].

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Key Terms

Term
Definition

An Example of the Trapezoidal Rule

• The trapezoidal rule approximates the area A of the region bound by the curve of a continuous function f (x) and the x-axis using N partitions on...

note

  • Here’s a strange situation. Suppose you are stranded on a desert island with a group of people, and for some reason you need to know the va...

Use the trapezoidal rule with 4 trapezoids to find an approximation for ∫2 0 1/1+x^2dx.

A≈287/260

Use the trapezoidal rule with 4 trapezoids of equal base to approximate ∫ 2 1 1/x^2dx.

A≈1/8[1+2(4/5)^2+2(2/3)^2+2(4/7)^`2+1/4]

Use the trapezoidal rule with 4 trapezoids to find an expression that approximates ∫4 2 e^x dx.

1/4(e^2+2e^5/2+2e^3+2e^7/2+e^4)

Use the trapezoidal rule with 3 trapezoids to find an approximation for ∫ 2 1 √x^2−1 dx.

A≈8+3√3+2√7/18

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TermDefinition

An Example of the Trapezoidal Rule

• The trapezoidal rule approximates the area A of the region bound by the curve of a continuous function f (x) and the x-axis using N partitions on [a, b].

note

  • Here’s a strange situation. Suppose you are stranded on a desert island with a group of people, and for some reason you need to know the value of the natural log of three. Hey, weirder things have happened!

  • Well, if you remember from Calculus I that the integral of 1/x is equal to the natural log function, you can construct an integral whose area is equal to exactly ln 3.

  • How could you use this information to determine the value of ln 3?

  • Since the only tools you have to compute numbers are what you know, then you will have to approximate the value of ln 3.

  • Here is the trapezoidal rule. If you use this rule, your
    approximation will be better than those of the other people stranded on the island.

  • Here, the interval [a, b] is divided into 4 regions.
    So f (x) = 1/x, N = 4, a = 1, and b = 3.

  • If you wanted an even more precise approximation, you could increase the number of rectangles used in the approximation.

  • Each partition has a base of 1/2. Start by finding what the x-values are at the partitions. Then find their corresponding f (x)-values. Plug these values into the trapezoidal rule.

  • Find a common denominator and add the fractions together.

  • Notice that your answer is very close to the actual value of ln 3.

  • Try doing this example with eight partitions to see if the approximation gets better. Also, compare the trapezoidal rule to the rectangular approximation.

  • Here is another example. This time, all that is given is a table of values. The function itself isn’t known, but the integral can still be approximated using the Trapezoidal Rule.

  • There are eight partitions and the endpoints are a = 2 and b = 5.2. The approximate value of the integral is 20.94, and this was obtained by using only the points in the table that lie on the curve.

Use the trapezoidal rule with 4 trapezoids to find an approximation for ∫2 0 1/1+x^2dx.

A≈287/260

Use the trapezoidal rule with 4 trapezoids of equal base to approximate ∫ 2 1 1/x^2dx.

A≈1/8[1+2(4/5)^2+2(2/3)^2+2(4/7)^`2+1/4]

Use the trapezoidal rule with 4 trapezoids to find an expression that approximates ∫4 2 e^x dx.

1/4(e^2+2e^5/2+2e^3+2e^7/2+e^4)

Use the trapezoidal rule with 3 trapezoids to find an approximation for ∫ 2 1 √x^2−1 dx.

A≈8+3√3+2√7/18

Use the Trapezoidal Rule and the following data to estimate the value of the integral ∫ 2.2 1 y dx.

8.36

Use the Trapezoidal Rule and the following data to estimate the value of the integral ∫ 2.8 0 y dx.

18.88

Use the trapezoidal rule with 3 trapezoids to find an approximation for ∫ 3 2 1/1−x dx.

A≈−7/10

Use the trapezoidal rule with 4 trapezoids to find an approximation for ∫ 2 0 x√x^2+4 dx.

A≈1/8[15+8√2+4√5+√17]

Use the trapezoidal rule with 4 trapezoids to find an approximation for ∫π 0 sinx dx.

A≈π(1+√2)/4

Use the trapezoidal rule with four trapezoids to find an expression that approximates ∫ 3 1 √x dx.

1/4[1+2√3/2+2√2+2√5/2+√3]

Use the trapezoidal rule with 4 trapezoids to find an approximation for ∫1 0 x^3/2dx.

A≈5+2√2+3√3/32

Use the trapezoidal rule with 3 trapezoids to find an approximation for ∫ 1 0 1−x/1+x dx.

A≈2/5