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Finding a volume

Find the volume of the solid of revolution obtained by revolving the region enclosed by the graph of f ( x ) = x 2 ( x 2 + 1 ) 2 and the x -axis over the interval [ 0 , 1 ] about the y -axis.

Let’s begin by sketching the region to be revolved (see [link] ). From the sketch, we see that the shell method is a good choice for solving this problem.

This figure is the graph of the function f(x) = x^2/(x^2+1)^2. It is a curve above the x-axis. It is decreasing in the second quadrant, intersects at the origin, and increases in the first quadrant. Between x = 0 and x = 1, there is shaded area under the curve.
We can use the shell method to find the volume of revolution obtained by revolving the region shown about the y -axis.

The volume is given by

V = 2 π 0 1 x · x 2 ( x 2 + 1 ) 2 d x = 2 π 0 1 x 3 ( x 2 + 1 ) 2 d x .

Since deg ( ( x 2 + 1 ) 2 ) = 4 > 3 = deg ( x 3 ) , we can proceed with partial fraction decomposition. Note that ( x 2 + 1 ) 2 is a repeated irreducible quadratic. Using the decomposition described in the problem-solving strategy, we get

x 3 ( x 2 + 1 ) 2 = A x + B x 2 + 1 + C x + D ( x 2 + 1 ) 2 .

Finding a common denominator and equating the numerators gives

x 3 = ( A x + B ) ( x 2 + 1 ) + C x + D .

Solving, we obtain A = 1 , B = 0 , C = −1 , and D = 0 . Substituting back into the integral, we have

V = 2 π 0 1 x 3 ( x 2 + 1 ) 2 d x = 2 π 0 1 ( x x 2 + 1 x ( x 2 + 1 ) 2 ) d x = 2 π ( 1 2 ln ( x 2 + 1 ) + 1 2 · 1 x 2 + 1 ) | 0 1 = π ( ln 2 1 2 ) .
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Set up the partial fraction decomposition for x 2 + 3 x + 1 ( x + 2 ) ( x 3 ) 2 ( x 2 + 4 ) 2 d x .

x 2 + 3 x + 1 ( x + 2 ) ( x 3 ) 2 ( x 2 + 4 ) 2 = A x + 2 + B x 3 + C ( x 3 ) 2 + D x + E x 2 + 4 + F x + G ( x 2 + 4 ) 2

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Key concepts

  • Partial fraction decomposition is a technique used to break down a rational function into a sum of simple rational functions that can be integrated using previously learned techniques.
  • When applying partial fraction decomposition, we must make sure that the degree of the numerator is less than the degree of the denominator. If not, we need to perform long division before attempting partial fraction decomposition.
  • The form the decomposition takes depends on the type of factors in the denominator. The types of factors include nonrepeated linear factors, repeated linear factors, nonrepeated irreducible quadratic factors, and repeated irreducible quadratic factors.

Express the rational function as a sum or difference of two simpler rational expressions.

1 ( x 3 ) ( x 2 )

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x 2 + 1 x ( x + 1 ) ( x + 2 )

2 x + 1 + 5 2 ( x + 2 ) + 1 2 x

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3 x 2 x 2 + 1 ( Hint: Use long division first.)

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2 x 4 x 2 2 x

2 x 2 + 4 x + 8 + 16 x 2

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1 ( x 1 ) ( x 2 + 1 )

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1 x 2 ( x 1 )

1 x 2 1 x + 1 x 1

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1 x ( x 1 ) ( x 2 ) ( x 3 )

1 2 ( x 2 ) + 1 2 ( x 1 ) 1 6 x + 1 6 ( x 3 )

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1 x 4 1 = 1 ( x + 1 ) ( x 1 ) ( x 2 + 1 )

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3 x 2 x 3 1 = 3 x 2 ( x 1 ) ( x 2 + x + 1 )

1 x 1 + 2 x + 1 x 2 + x + 1

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3 x 4 + x 3 + 20 x 2 + 3 x + 31 ( x + 1 ) ( x 2 + 4 ) 2

2 x + 1 + x x 2 + 4 1 ( x 2 + 4 ) 2

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Use the method of partial fractions to evaluate each of the following integrals.

d x ( x 3 ) ( x 2 )

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3 x x 2 + 2 x 8 d x

ln | 2 x | + 2 ln | 4 + x | + C

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x x 2 4 d x

1 2 ln | 4 x 2 | + C

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d x x ( x 1 ) ( x 2 ) ( x 3 )

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2 x 2 + 4 x + 22 x 2 + 2 x + 10 d x

2 ( x + 1 3 arctan ( 1 + x 3 ) ) + C

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2 x x 2 + x d x

2 ln | x | 3 ln | 1 + x | + C

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2 x 2 x 6 d x

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d x x 3 2 x 2 4 x + 8

1 16 ( 4 −2 + x ln | −2 + x | + ln | 2 + x | ) + C

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d x x 4 10 x 2 + 9

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Evaluate the following integrals, which have irreducible quadratic factors.

2 ( x 4 ) ( x 2 + 2 x + 6 ) d x

1 30 ( −2 5 arctan [ 1 + x 5 ] + 2 ln | −4 + x | ln | 6 + 2 x + x 2 | ) + C

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

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x 3 + 6 x 2 + 3 x + 6 x 3 + 2 x 2 d x

3 x + 4 ln | x + 2 | + x + C

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x ( x 1 ) ( x 2 + 2 x + 2 ) 2 d x

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Use the method of partial fractions to evaluate the following integrals.

3 x + 4 ( x 2 + 4 ) ( 3 x ) d x

ln | 3 x | + 1 2 ln | x 2 + 4 | + C

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2 ( x + 2 ) 2 ( 2 x ) d x

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3 x + 4 x 3 2 x 4 d x ( Hint: Use the rational root theorem.)

ln | x 2 | 1 2 ln | x 2 + 2 x + 2 | + C

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Use substitution to convert the integrals to integrals of rational functions. Then use partial fractions to evaluate the integrals.

0 1 e x 36 e 2 x d x (Give the exact answer and the decimal equivalent. Round to five decimal places.)

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e x d x e 2 x e x d x

x + ln | 1 e x | + C

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sin x d x 1 cos 2 x

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sin x cos 2 x + cos x 6 d x

1 5 ln | cos x + 3 cos x 2 | + C

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d t ( e t e t ) 2

1 2 2 e 2 t + C

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1 + e x 1 e x d x

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d x 1 + x + 1

2 1 + x 2 ln | 1 + 1 + x | + C

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cos x sin x ( 1 sin x ) d x

ln | sin x 1 sin x | + C

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e x ( e 2 x 4 ) 2 d x

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1 2 1 x 2 4 x 2 d x

3 4

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1 1 + e x d x

x ln ( 1 + e x ) + C

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Use the given substitution to convert the integral to an integral of a rational function, then evaluate.

1 t t 3 d t t = x 3

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1 x + x 3 d x ; x = u 6

6 x 1 / 6 3 x 1 / 3 + 2 x 6 ln ( 1 + x 1 / 6 ) + C

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Graph the curve y = x 1 + x over the interval [ 0 , 5 ] . Then, find the area of the region bounded by the curve, the x -axis, and the line x = 4 .

This figure is a graph of the function y = x/(1 + x). The graph is only in the first quadrant. It begins at the origin and increases into the first quadrant. The curve stops at x = 5.
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Find the volume of the solid generated when the region bounded by y = 1 / x ( 3 x ) , y = 0 , x = 1 , and x = 2 is revolved about the x- axis.

4 3 π arctanh [ 1 3 ] = 1 3 π ln 4

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The velocity of a particle moving along a line is a function of time given by v ( t ) = 88 t 2 t 2 + 1 . Find the distance that the particle has traveled after t = 5 sec.

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Solve the initial-value problem for x as a function of t.

( t 2 7 t + 12 ) d x d t = 1 , ( t > 4 , x ( 5 ) = 0 )

x = ln | t 3 | + ln | t 4 | + ln 2

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( t + 5 ) d x d t = x 2 + 1 , t > 5 , x ( 1 ) = tan 1

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( 2 t 3 2 t 2 + t 1 ) d x d t = 3 , x ( 2 ) = 0

x = ln | t 1 | 2 arctan ( 2 t ) 1 2 ln ( t 2 + 1 2 ) + 2 arctan ( 2 2 ) + 1 2 ln 4.5

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Find the x -coordinate of the centroid of the area bounded by

y ( x 2 9 ) = 1 , y = 0 , x = 4 , and x = 5 . (Round the answer to two decimal places.)

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Find the volume generated by revolving the area bounded by y = 1 x 3 + 7 x 2 + 6 x x = 1 , x = 7 , and y = 0 about the y -axis.

2 5 π ln 28 13

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Find the area bounded by y = x 12 x 2 8 x 20 , y = 0 , x = 2 , and x = 4 . (Round the answer to the nearest hundredth.)

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Evaluate the integral d x x 3 + 1 .

arctan [ −1 + 2 x 3 ] 3 + 1 3 ln | 1 + x | 1 6 ln | 1 x + x 2 | + C

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For the following problems, use the substitutions tan ( x 2 ) = t , d x = 2 1 + t 2 d t , sin x = 2 t 1 + t 2 , and cos x = 1 t 2 1 + t 2 .

Find the area under the curve y = 1 1 + sin x between x = 0 and x = π . (Assume the dimensions are in inches.)

2.0 in. 2

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Given tan ( x 2 ) = t , derive the formulas d x = 2 1 + t 2 d t , sin x = 2 t 1 + t 2 , and cos x = 1 t 2 1 + t 2 .

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Evaluate x 8 3 x d x .

3 ( −8 + x ) 1 / 3
−2 3 arctan [ −1 + ( −8 + x ) 1 / 3 3 ]
−2 ln [ 2 + ( −8 + x ) 1 / 3 ]
+ ln [ 4 2 ( −8 + x ) 1 / 3 + ( −8 + x ) 2 / 3 ] + C

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Practice Key Terms 1

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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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