## 7.2. Integration Techniques

### Example 7.2.10: Application of the Trapezoid Rule

Compare the numeric approximations to the integral

First, let's determine the exact value of the integral by substitution. Let
obtained by using (a) a left Riemann sum and (b) the Trapezoid Rule, using a partition of size 5 and of size 100.sin(x) cos(x) dx

Thenu = sin(x)so that

du/dx = cos(x)ordu = cos(x) dx

sin(x) cos(x) dx = u du =

= 1/2 (sin^{2}(1) - sin^{2}(0)) = 0.5(0.7080734183 - 0) = 0.3540367092

To find the left Riemann sum, we let f(x) = sin(x) cos(x) and compute:
and therefore |

The error between the approximate and exact value is about 0.05, or 14%. To estimate the error using the trapezoid rule, we computeR(P, f) = 1/5*(f(0) + f(0.2) + f(0.4) + f(0.6) + f(0.8)) =

= 0.2 * 1.519193562 = 0.3038387122

*f''(x) = -4 sin(x) cos(x)*so that

*K = 4*. Then

so that even the theoretically worst error is a lot better (less than 4%). To find the value using the trapezoid rule, we need to evaluate|R| < 4/12 * 1 * 0.2^{2}= 0.04 / 3 = 0.013

*f*at the same values as before and also compute

*f(1) = 0.4546487134*. Then the trapezoid rule, combining these numbers a little different than the left Riemann sum, gives:

which is indeed much closer than our previous approximation (the error is only about 1%).[1/2 f(0) + f(0.2) + f(0.4) + f(0.6) + f(0.8) + 1/2 f(1)] * 0.2 =

= [0 + 1.519193562 + 0.5 * 0.4546487134 ] * 0.2 =

= 0.3493035838

Incidentally, if we increased the size of the partition to 100, the error
committed by the trapezoid rule would already be less than
1/3 * 0.01 or 0.01%, whereas the left
Riemann sum still has an error of about 0.003, or about 1% (compare the
value of the applet with the exact value above).
^{2} = 0.00003 |