APEX Arc Length and Surface Area

Section 7.4 Arc Length and Surface Area

In previous sections we have used integration to answer the following questions:

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Given a region, what is its area?
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Given a solid, what is its volume?
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Figure 7.4.1. Video introduction to Section 7.4

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In this section, we address two related questions:

Given a curve, what is its length? This is often referred to as arc length.
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Given a solid, what is its surface area?
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Subsection 7.4.1 Arc Length

Consider the graph of

y = \sin (x)

[0, π]

given in Figure 7.4.2.(a). How long is this curve? That is, if we were to use a piece of string to exactly match the shape of this curve, how long would the string be?

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As we have done in the past, we start by approximating; later, we will refine our answer using limits to get an exact solution.

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The length of straight-line segments is easy to compute using the Distance Formula. We can approximate the length of the given curve by approximating the curve with straight lines and measuring their lengths.

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Graph of the sine function for x between 0 and pi. — (a)
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Graph of the sine function for x between 0 and pi and four straight lines approximating this curve. — (a)
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In Figure 7.4.2.(b), the curve

y = \sin (x)

has been approximated with 4 line segments (the interval

[0, π]

has been divided into 4 subintervals of equal length). It is clear that these four line segments approximate

y = \sin (x)

very well on the first and last subinterval, though not so well in the middle. Regardless, the sum of the lengths of the line segments is

3.79,

so we approximate the arc length of

y = \sin (x)

[0, π]

to be

3.79 .

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In general, we can approximate the arc length of

y = f (x)

[a, b]

in the following manner. Let

a = x_{0} < x_{1} < \dots < x_{n - 1} < x_{n} = b

be a partition of

[a, b]

into

n

subintervals. Let

Δ x_{i}

represent the length of the

i

th subinterval

[x_{i - 1}, x_{i}] .

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Graph of a portion of a curve with a line between the start and endpoint which is used to approximate the length of the curve. — Figure 7.4.3. Zooming in on the $i$ th subinterval $[x_{i - 1}, x_{i}$ ] of a partition of $[a, b]$
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Figure 7.4.3 zooms in on the

i

th subinterval where

y = f (x)

is approximated by a straight line segment. The dashed lines show that we can view this line segment as the hypotenuse of a right triangle whose sides have length

Δ x_{i}

and

Δ y_{i} .

Using the Pythagorean Theorem, the length of this line segment is

\sqrt{Δ x_{i}^{2} + Δ y_{i}^{2}} .

Summing over all subintervals gives an arc length approximation

L \approx \sum_{i = 1}^{n} \sqrt{Δ x_{i}^{2} + Δ y_{i}^{2}} .

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As shown here, this is not a Riemann Sum. While we could conclude that taking a limit as the subinterval length goes to zero gives the exact arc length, we would not be able to compute the answer with a definite integral. We need first to do a little algebra.

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In the above expression factor out a

Δ x_{i}^{2}

term:

\begin{aligned} \sum_{i = 1}^{n} \sqrt{Δ x_{i}^{2} + Δ y_{i}^{2}} & = \sum_{i = 1}^{n} \sqrt{Δ x_{i}^{2} (1 + \frac{Δ y_{i}^{2}}{Δ x_{i}^{2}})} . \end{aligned}

Now pull the

Δ x_{i}^{2}

term out of the square root:

\begin{aligned} = \sum_{i = 1}^{n} \sqrt{1 + \frac{Δ y_{i}^{2}}{Δ x_{i}^{2}}} Δ x_{i} . \end{aligned}

This is nearly a Riemann Sum. Consider the

Δ y_{i}^{2} / Δ x_{i}^{2}

term. The expression

Δ y_{i} / Δ x_{i}

measures the “change in

y

/change in

x,

” that is, the “rise over run” of

f

on the

i

th subinterval. The Mean Value Theorem of Differentiation (Theorem 3.2.4) states that there is a

c_{i}

in the

i

th subinterval where

f^{'} (c_{i}) = Δ y_{i} / Δ x_{i} .

Thus we can rewrite our above expression as:

\begin{aligned} = \sum_{i = 1}^{n} \sqrt{1 + f^{'} (c_{i})^{2}} Δ x_{i} . \end{aligned}

This is a Riemann Sum. As long as

f^{'}

is continuous, we can invoke Theorem 5.3.26 and conclude

\begin{aligned} = \int_{a}^{b} \sqrt{1 + f^{'} (x)^{2}} d x . \end{aligned}

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Theorem 7.4.4. Arc Length.

Let

f

be differentiable on

[a, b],

where

f^{'}

is also continuous on

[a, b] .

Then the arc length of

f

from

x = a

x = b

L = \int_{a}^{b} \sqrt{1 + f^{'} (x)^{2}} d x .

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As the integrand contains a square root, it is often difficult to use the formula in Theorem 7.4.4 to find the length exactly. When exact answers are difficult to come by, we resort to using numerical methods of approximating definite integrals. The following examples will demonstrate this.

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Example 7.4.5. Finding arc length.

Find the arc length of

f (x) = x^{3 / 2}

from

x = 0

x = 4 .

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Solution 1.

We find

f^{'} (x) = \frac{3}{2} x^{1 / 2};

note that on

[0, 4],

f

is differentiable and

f^{'}

is also continuous. Using the formula, we find the arc length

L

\begin{aligned} L & = \int_{0}^{4} \sqrt{1 + {(\frac{3}{2} x^{1 / 2})}^{2}} d x \\ = \int_{0}^{4} \sqrt{1 + \frac{9}{4} x} d x \\ = \int_{0}^{4} {(1 + \frac{9}{4} x)}^{1 / 2} d x \\ = \frac{2}{3} \cdot \frac{4}{9} \cdot {(1 + \frac{9}{4} x)}^{3 / 2} |_{0}^{4} \\ = \frac{8}{27} (10^{3 / 2} - 1) \approx 9.07 units . \end{aligned}

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Graph of the function from the example. — Figure 7.4.6. A graph of $f (x) = x^{3 / 2}$ from Example 7.4.5
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A graph of

f

is given in Figure 7.4.6.

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Solution 2. Video solution

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Example 7.4.7. Finding arc length.

Find the arc length of

f (x) = \frac{1}{8} x^{2} - \ln (x)

from

x = 1

x = 2 .

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Solution 1.

This function was chosen specifically because the resulting integral can be evaluated exactly. We begin by finding

f^{'} (x) = x / 4 - 1 / x .

The arc length is

\begin{aligned} L & = \int_{1}^{2} \sqrt{1 + {(\frac{x}{4} - \frac{1}{x})}^{2}} d x \\ = \int_{1}^{2} \sqrt{1 + \frac{x^{2}}{16} - \frac{1}{2} + \frac{1}{x^{2}}} d x \\ = \int_{1}^{2} \sqrt{\frac{x^{2}}{16} + \frac{1}{2} + \frac{1}{x^{2}}} d x \\ = \int_{1}^{2} \sqrt{{(\frac{x}{4} + \frac{1}{x})}^{2}} d x \end{aligned}

\begin{aligned} = \int_{1}^{2} (\frac{x}{4} + \frac{1}{x}) d x \\ = {(\frac{x^{2}}{8} + \ln (x)) |}_{1}^{2} \\ = \frac{3}{8} + \ln (2) \approx 1.07 units . \end{aligned}

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A graph of

f

is given in Figure 7.4.8; the portion of the curve measured in this problem is in bold.

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Solution 2. Video solution

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The previous examples found the arc length exactly through careful choice of the functions. In general, exact answers are much more difficult to come by and numerical approximations are necessary.

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Example 7.4.9. Approximating arc length numerically.

Find the length of the sine curve from

x = 0

x = π .

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Solution.

This is somewhat of a mathematical curiosity; in Example 5.4.14 we found the area under one “hump” of the sine curve is 2 square units; now we are measuring its arc length.

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The setup is straightforward:

f (x) = \sin (x)

and

f^{'} (x) = \cos (x) .

Thus

L = \int_{0}^{π} \sqrt{1 + \cos^{2} (x)} d x .

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This integral cannot be evaluated in terms of elementary functions so we will approximate it with Simpson’s Method with

n = 4 .

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$x$	$\sqrt{1 + \cos^{2} (x)}$
$0$	$\sqrt{2}$
$π / 4$	$\sqrt{3 / 2}$
$π / 2$	$1$
$3 π / 4$	$\sqrt{3 / 2}$
$π$	$\sqrt{2}$

Figure 7.4.10. A table of values of

y = \sqrt{1 + \cos^{2} (x)}

to evaluate a definite integral in Example 7.4.9

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Figure 7.4.10 gives

\sqrt{1 + \cos^{2} (x)}

evaluated at 5 evenly spaced points in

[0, π] .

Simpson’s Rule then states that

\begin{aligned} \int_{0}^{π} \sqrt{1 + \cos^{2} (x)} d x & \approx \frac{π - 0}{4 \cdot 3} (\sqrt{2} + 4 \sqrt{3 / 2} + 2 (1) + 4 \sqrt{3 / 2} + \sqrt{2}) \\ = 3.82918 . \end{aligned}

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Using a computer with

n = 100

the approximation is

L \approx 3.8202;

our approximation with

n = 4

is quite good.

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Subsection 7.4.2 Surface Area of Solids of Revolution

We have already seen how a curve

y = f (x)

[a, b]

can be revolved around an axis to form a solid. Instead of computing its volume, we now consider its surface area.

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Figure 7.4.11. Video introduction to Subsection 7.4.2

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Graph of an arbitrary function on the interval from a to b, with a line approximating a small portion of the curve. — (a)
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We begin as we have in the previous sections: we partition the interval

[a, b]

with

n

subintervals, where the

i

th subinterval is

[x_{i - 1}, x_{i}] .

On each subinterval, we can approximate the curve

y = f (x)

with a straight line that connects

f (x_{i - 1})

and

f (x_{i})

as shown in Figure 7.4.12.(a). Revolving this line segment about the

x

-axis creates part of a cone (called a frustum of a cone) as shown in Figure 7.4.12.(b). The surface area of a frustum of a cone is

2 π \cdot length \cdot average of the two radii R and r .

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The length is given by

L;

we use the material just covered by arc length to state that

L \approx \sqrt{1 + f^{'} (c_{i})^{2}} Δ x_{i}

for some

c_{i}

in the

i

th subinterval. The radii are just the function evaluated at the endpoints of the interval. That is,

R = f (x_{i}) and r = f (x_{i - 1}) .

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Thus the surface area of this sample frustum of the cone is approximately

2 π \frac{f (x_{i - 1}) + f (x_{i})}{2} \sqrt{1 + f^{'} (c_{i})^{2}} Δ x_{i} .

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Since

f

is a continuous function, the Intermediate Value Theorem states there is some

d_{i}

[x_{i - 1}, x_{i}]

such that

f (d_{i}) = \frac{f (x_{i - 1}) + f (x_{i})}{2};

we can use this to rewrite the above equation as

2 π f (d_{i}) \sqrt{1 + f^{'} (c_{i})^{2}} Δ x_{i} .

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Summing over all the subintervals we get the total surface area to be approximately

Surface Area \approx \sum_{i = 1}^{n} 2 π f (d_{i}) \sqrt{1 + f^{'} (c_{i})^{2}} Δ x_{i},

which is a Riemann Sum. Taking the limit as the subinterval lengths go to zero gives us the exact surface area, given in the following theorem.

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Theorem 7.4.13. Surface Area of a Solid of Revolution.

Let

f

be differentiable on

[a, b],

where

f^{'}

is also continuous on

[a, b] .

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The surface area of the solid formed by revolving the graph of $y = f (x),$ where $f (x) \geq 0,$ about the $x$ -axis is

$Surface Area = 2 π \int_{a}^{b} f (x) \sqrt{1 + f^{'} (x)^{2}} d x .$

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The surface area of the solid formed by revolving the graph of $y = f (x)$ about the $y$ -axis, where $a, b \geq 0,$ is

$Surface Area = 2 π \int_{a}^{b} x \sqrt{1 + f^{'} (x)^{2}} d x .$

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(When revolving

y = f (x)

about the

y

-axis, the radii of the resulting frustum are

x_{i - 1}

and

x_{i};

their average value is simply the midpoint of the interval. In the limit, this midpoint is just

x .

This gives the second part of Theorem 7.4.13.)

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Example 7.4.14. Finding surface area of a solid of revolution.

Find the surface area of the solid formed by revolving

y = \sin (x)

[0, π]

around the

x

-axis, as shown in Figure 7.4.15.

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Link to full-sized image

Figure 7.4.15. Revolving

y = \sin (x)

[0, π]

about the

x

-axis

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Solution 1.

The setup is relatively straightforward. Using Theorem 7.4.13, we have the surface area

S A

is:

\begin{aligned} S A & = 2 π \int_{0}^{π} \sin (x) \sqrt{1 + \cos^{2} (x)} d x \\ = - 2 π \frac{1}{2} {(\sinh^{- 1} (\cos (x)) + \cos (x) \sqrt{1 + \cos^{2} (x)}) |}_{0}^{π} \\ = 2 π (\sqrt{2} + \sinh^{- 1} (1)) \\ \approx 14.42 {units}^{2} . \end{aligned}

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The integration step above is nontrivial, utilizing the integration method of Trigonometric Substitution from Section 6.4.

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It is interesting to see that the surface area of a solid, whose shape is defined by a trigonometric function, involves both a square root and an inverse hyperbolic trigonometric function.

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Solution 2. Video solution

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Example 7.4.16. Finding surface area of a solid of revolution.

Find the surface area of the solid formed by revolving the curve

y = x^{2}

[0, 1]

about:

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the $x$ -axis
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the $y$ -axis.
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Link to full-sized image

(a)

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Link to full-sized image

(b)

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Figure 7.4.17. The solids used in Example 7.4.16

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Solution 1.

The integral is straightforward to setup:

$\begin{aligned} S A & = 2 π \int_{0}^{1} x^{2} \sqrt{1 + (2 x)^{2}} d x . \end{aligned}$
Like the integral in Example 7.4.14, this requires Trigonometric Substitution.
$\begin{aligned} = {\frac{π}{32} (2 (8 x^{3} + x) \sqrt{1 + 4 x^{2}} - \sinh^{- 1} (2 x)) |}_{0}^{1} \\ = \frac{π}{32} (18 \sqrt{5} - \sinh^{- 1} (2)) \\ \approx 3.81 {units}^{2} . \end{aligned}$

The solid formed by revolving $y = x^{2}$ around the $x$ -axis is graphed in Figure 7.4.17.(a).

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Since we are revolving around the $y$ -axis, the “radius” of the solid is not $f (x)$ but rather $x .$ Thus the integral to compute the surface area is:

$\begin{aligned} S A & = 2 π \int_{0}^{1} x \sqrt{1 + (2 x)^{2}} d x . \end{aligned}$
This integral can be solved using substitution. Set $u = 1 + 4 x^{2};$ the new bounds are $u = 1$ to $u = 5 .$ We then have
$\begin{aligned} = \frac{π}{4} \int_{1}^{5} \sqrt{u} d u \\ = {\frac{π}{4} \frac{2}{3} u^{3 / 2} |}_{1}^{5} \\ = \frac{π}{6} (5 \sqrt{5} - 1) \\ \approx 5.33 {units}^{2} . \end{aligned}$

The solid formed by revolving $y = x^{2}$ about the $y$ -axis is graphed in Figure 7.4.17.(b).

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Solution 2. Video solution

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Our final example is a famous mathematical “paradox.”

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Example 7.4.18. The surface area and volume of Gabriel’s Horn.

Consider the solid formed by revolving

y = 1 / x

about the

x

-axis on

[1, \infty) .

Find the volume and surface area of this solid. (This shape, as graphed in Figure 7.4.19, is known as “Gabriel’s Horn” since it looks like a very long horn that only a supernatural person, such as an angel, could play.)

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Link to full-sized image

Figure 7.4.19. A graph of Gabriel’s Horn

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Solution 1.

To compute the volume it is natural to use the Disk Method. We have:

\begin{aligned} V & = π \int_{1}^{\infty} \frac{1}{x^{2}} d x \\ = lim_{b \to \infty} π \int_{1}^{b} \frac{1}{x^{2}} d x \\ = lim_{b \to \infty} {π (\frac{- 1}{x}) |}_{1}^{b} \\ = lim_{b \to \infty} π (1 - \frac{1}{b}) \\ = π {units}^{3} . \end{aligned}

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Gabriel’s Horn has a finite volume of

π

cubic units. Since we have already seen that regions with infinite length can have a finite area, this is not too difficult to accept.

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We now consider its surface area. The integral is straightforward to setup:

\begin{aligned} S A & = 2 π \int_{1}^{\infty} \frac{1}{x} \sqrt{1 + 1 / x^{4}} d x . \end{aligned}

Integrating this expression is not trivial. We can, however, compare it to other improper integrals. Since

1 < \sqrt{1 + 1 / x^{4}}

[1, \infty),

we can state that

\begin{aligned} 2 π \int_{1}^{\infty} \frac{1}{x} d x & < 2 π \int_{1}^{\infty} \frac{1}{x} \sqrt{1 + 1 / x^{4}} d x . \end{aligned}

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By Key Idea 6.8.17, the improper integral on the left diverges. Since the integral on the right is larger, we conclude it also diverges, meaning Gabriel’s Horn has infinite surface area.

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Hence the “paradox”: we can fill Gabriel’s Horn with a finite amount of paint, but since it has infinite surface area, we can never paint it.

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Somehow this paradox is striking when we think about it in terms of volume and area. However, we have seen a similar paradox before, as referenced above. We know that the area under the curve

y = 1 / x^{2}

[1, \infty)

is finite, yet the shape has an infinite perimeter. Strange things can occur when we deal with the infinite.

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Solution 2. Video solution

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A standard equation from physics is “Work = force × distance”, when the force applied is constant. In Section 7.5 we learn how to compute work when the force applied is variable.

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Exercises 7.4.3 Exercises

Terms and Concepts

1.

The integral formula for computing arc length was found by first approximating arc length with straight line segments.

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True.
False.

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2.

The integral formula for computing arc length includes a square root, meaning the integration is probably easy.

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True.
False.

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Problems

Exercise Group.

[0, π / 4] .

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Exercise Group.

[- π / 4, π / 4] .

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Exercise Group.

In the following exercises, use Simpson’s Rule, with

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29.

Find the length of the curve

f (x) = \sqrt{x}

[0, 1] .

(Note: this is the same as the length of

f (x) = x^{2}

[0, 1] .

Why?)

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30.

Find the length of the curve

f (x) = \sqrt{1 - x^{2}}

[- 1, 1] .

(Note: this describes the top half of a circle with radius 1.)

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31.

Find the length of the curve

f (x) = \sqrt{1 - x^{2} / 9}

[- 3, 3] .

(Note: this describes the top half of an ellipse with a major axis of length 6 and a minor axis of length 2.)

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32.

Find the surface area of the solid formed by revolving

y = \sqrt{x}

[0, 1]

about the

x

-axis.

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33.

Find the surface area of the sphere formed by revolving

y = \sqrt{1 - x^{2}}

[- 1, 1]

about the

x

-axis.

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