James Stewart-Calculus_ Early Transcendentals-Cengage Learning (2015)

516 Chapter 7 Techniques of Integration
y
Trapezoidal Rule
y b
f sxd dx < T n − Dx
a
2 f f sx 0d 1 2 f sx 1 d 1 2 f sx 2 d 1 ∙ ∙ ∙ 1 2 f sx n21 d 1 f sx n dg
where Dx − sb 2 adyn and x i − a 1 i Dx.
0
x¸ ⁄ ¤ ‹ x¢
FIGURE 2
Trapezoidal approximation
1 2
FIGURE 3
1 2
FIGURE 4
1
y=
x
1
y=
x
x
The reason for the name Trapezoidal Rule can be seen from Figure 2, which illustrates
the case with f sxd > 0 and n − 4. The area of the trapezoid that lies above the ith subinterval
is
Dx S f sx i21d 1 f sx i d
2
D − Dx
2 f f sx i21d 1 f sx i dg
and if we add the areas of all these trapezoids, we get the right side of the Trapezoidal
Rule.
Example 1 Use (a) the Trapezoidal Rule and (b) the Midpoint Rule with n − 5 to
approximate the integral y 2 s1yxd dx.
1
SOLUTION
(a) With n − 5, a − 1, and b − 2, we have Dx − s2 2 1dy5 − 0.2, and so the Trapezoidal
Rule gives
y 2
1
1
x dx < T 5 − 0.2 f f s1d 1 2 f s1.2d 1 2 f s1.4d 1 2 f s1.6d 1 2 f s1.8d 1 f s2dg
2
− 0.1S 1 1 1 2
1.2 1 2
1.4 1 2
1.6 1 2
1.8 2D 1 1
< 0.695635
This approximation is illustrated in Figure 3.
(b) The midpoints of the five subintervals are 1.1, 1.3, 1.5, 1.7, and 1.9, so the Midpoint
Rule gives
y 2
1
1
dx < Dx f f s1.1d 1 f s1.3d 1 f s1.5d 1 f s1.7d 1 f s1.9dg
x
− S 1 1
5 1.1 1 1
1.3 1 1
1.5 1 1
1.7 1.9D 1 1
< 0.691908
This approximation is illustrated in Figure 4.
n
y b
f sxd dx − approximation 1 error
a
In Example 1 we deliberately chose an integral whose value can be computed explicitly
so that we can see how accurate the Trapezoidal and Midpoint Rules are. By the
Fundamental Theorem of Calculus,
y 2
1
1
x dx − ln xg 2
1 − ln 2 − 0.693147 . . .
The error in using an approximation is defined to be the amount that needs to be added
to the approximation to make it exact. From the values in Example 1 we see that the
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