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Key concepts

  • Integrals of functions over infinite intervals are defined in terms of limits.
  • Integrals of functions over an interval for which the function has a discontinuity at an endpoint may be defined in terms of limits.
  • The convergence or divergence of an improper integral may be determined by comparing it with the value of an improper integral for which the convergence or divergence is known.

Key equations

  • Improper integrals
    a + f ( x ) d x = lim t + a t f ( x ) d x b f ( x ) d x = lim t t b f ( x ) d x + f ( x ) d x = 0 f ( x ) d x + 0 + f ( x ) d x

Evaluate the following integrals. If the integral is not convergent, answer “divergent.”

2 4 d x ( x 3 ) 2

divergent

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0 2 1 4 x 2 d x

π 2

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1 x e x d x

2 e

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x x 2 + 1 d x

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Without integrating, determine whether the integral 1 1 x 3 + 1 d x converges or diverges by comparing the function f ( x ) = 1 x 3 + 1 with g ( x ) = 1 x 3 .

Converges

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Without integrating, determine whether the integral 1 1 x + 1 d x converges or diverges.

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Determine whether the improper integrals converge or diverge. If possible, determine the value of the integrals that converge.

0 e x cos x d x

Converges to 1/2.

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1 x 2 + 1 d x

π

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−2 2 d x ( 1 + x ) 2

diverges

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0 sin x d x

diverges

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e x 1 + e 2 x d x

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−1 2 d x x 3

diverges

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0 3 1 x 1 d x

diverges

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3 5 5 ( x 4 ) 2 d x

diverges

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Determine the convergence of each of the following integrals by comparison with the given integral. If the integral converges, find the number to which it converges.

1 d x x 2 + 4 x ; compare with 1 d x x 2 .

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1 d x x + 1 ; compare with 1 d x 2 x .

Both integrals diverge.

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Evaluate the integrals. If the integral diverges, answer “diverges.”

0 1 d x x π

diverges

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0 1 d x 1 x

diverges

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0 d x x 2 + 1

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−1 1 d x 1 x 2

π

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0 e ln ( x ) d x

0.0

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x ( x 2 + 1 ) 2 d x

0.0

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Evaluate the improper integrals. Each of these integrals has an infinite discontinuity either at an endpoint or at an interior point of the interval.

0 3 d x 9 x 2

π 2

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6 24 d t t t 2 36

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0 4 x ln ( 4 x ) d x

8 ln ( 16 ) 4

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Evaluate .5 t d x 1 x 2 . (Be careful!) (Express your answer using three decimal places.)

1.047

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Evaluate 1 4 d x x 2 1 . (Express the answer in exact form.)

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Evaluate 2 d x ( x 2 1 ) 3 / 2 .

−1 + 2 3

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Find the area of the region in the first quadrant between the curve y = e −6 x and the x -axis.

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Find the area of the region bounded by the curve y = 7 x 2 , the x -axis, and on the left by x = 1 .

7.0

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Find the area under the curve y = 1 ( x + 1 ) 3 / 2 , bounded on the left by x = 3 .

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Find the area under y = 5 1 + x 2 in the first quadrant.

5 π 2

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Find the volume of the solid generated by revolving about the x -axis the region under the curve y = 3 x from x = 1 to x = .

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Find the volume of the solid generated by revolving about the y -axis the region under the curve y = 6 e −2 x in the first quadrant.

3 π

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Find the volume of the solid generated by revolving about the x -axis the area under the curve y = 3 e x in the first quadrant.

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The Laplace transform of a continuous function over the interval [ 0 , ) is defined by F ( s ) = 0 e s x f ( x ) d x (see the Student Project). This definition is used to solve some important initial-value problems in differential equations, as discussed later. The domain of F is the set of all real numbers s such that the improper integral converges. Find the Laplace transform F of each of the following functions and give the domain of F .

f ( x ) = cos ( 2 x )

s s 2 + 4 , s > 0

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Use the formula for arc length to show that the circumference of the circle x 2 + y 2 = 1 is 2 π .

Answers will vary.

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A function is a probability density function if it satisfies the following definition: f ( t ) d t = 1 . The probability that a random variable x lies between a and b is given by P ( a x b ) = a b f ( t ) d t .

Show that f ( x ) = { 0 if x < 0 7 e −7 x if x 0 is a probability density function.

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Find the probability that x is between 0 and 0.3. (Use the function defined in the preceding problem.) Use four-place decimal accuracy.

0.8775

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Chapter review exercises

For the following exercises, determine whether the statement is true or false. Justify your answer with a proof or a counterexample.

e x sin ( x ) d x cannot be integrated by parts.

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1 x 4 + 1 d x cannot be integrated using partial fractions.

False

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In numerical integration, increasing the number of points decreases the error.

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Integration by parts can always yield the integral.

False

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For the following exercises, evaluate the integral using the specified method.

x 2 sin ( 4 x ) d x using integration by parts

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1 x 2 x 2 + 16 d x using trigonometric substitution

x 2 + 16 16 x + C

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x ln ( x ) d x using integration by parts

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3 x x 3 + 2 x 2 5 x 6 d x using partial fractions

1 10 ( 4 ln ( 2 x ) + 5 ln ( x + 1 ) 9 ln ( x + 3 ) ) + C

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x 5 ( 4 x 2 + 4 ) 5 / 2 d x using trigonometric substitution

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4 sin 2 ( x ) sin 2 ( x ) cos ( x ) d x using a table of integrals or a CAS

4 sin 2 ( x ) sin ( x ) x 2 + C

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For the following exercises, integrate using whatever method you choose.

sin 2 ( x ) cos 2 ( x ) d x

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x 3 x 2 + 2 d x

1 15 ( x 2 + 2 ) 3 / 2 ( 3 x 2 4 ) + C

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3 x 2 + 1 x 4 2 x 3 x 2 + 2 x d x

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1 x 4 + 4 d x

1 16 ln ( x 2 + 2 x + 2 x 2 2 x + 2 ) 1 8 tan −1 ( 1 x ) + 1 8 tan −1 ( x + 1 ) + C

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For the following exercises, approximate the integrals using the midpoint rule, trapezoidal rule, and Simpson’s rule using four subintervals, rounding to three decimals.

[T] 1 2 x 5 + 2 d x

M 4 = 3.312 , T 4 = 3.354 , S 4 = 3.326

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[T] 0 π e sin ( x 2 ) d x

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[T] 1 4 ln ( 1 / x ) x d x

M 4 = −0.982 , T 4 = −0.917 , S 4 = −0.952

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For the following exercises, evaluate the integrals, if possible.

1 1 x n d x , for what values of n does this integral converge or diverge?

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1 e x x d x

approximately 0.2194

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For the following exercises, consider the gamma function given by Γ ( a ) = 0 e y y a 1 d y .

Show that Γ ( a ) = ( a 1 ) Γ ( a 1 ) .

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Extend to show that Γ ( a ) = ( a 1 ) ! , assuming a is a positive integer.

The fastest car in the world, the Bugati Veyron, can reach a top speed of 408 km/h. The graph represents its velocity.

This figure has a graph in the first quadrant. It increases to where x is approximately 03:00 mm:ss and then drops off steep. The maximum height of the graph, here the drop occurs is approximately 420 km/h.
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[T] Use the graph to estimate the velocity every 20 sec and fit to a graph of the form v ( t ) = a exp b x sin ( c x ) + d . ( Hint: Consider the time units.)

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[T] Using your function from the previous problem, find exactly how far the Bugati Veyron traveled in the 1 min 40 sec included in the graph.

Answers may vary. Ex: 9.405 km

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Source:  OpenStax, Calculus volume 2. OpenStax CNX. Feb 05, 2016 Download for free at http://cnx.org/content/col11965/1.2
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