# Simpson's Rule/Trapezoidal Approximation - Error rate help

1. May 5, 2010

### Asphyxiated

1. The problem statement, all variables and given/known data

$$\int^{ \pi}_{0} sin(x)dx \;\;\;\;\;\;\;\; dx=\frac{ \pi}{2}$$

2. Relevant equations

Trapezoidal Approximation:

$$|f''(x)| \leq M \;\;\;\;\; for \;\;\;\;\; a \leq x \leq b$$

$$\frac {b-a}{12}(M)(dx)^{2} = Error$$

Simpson's Rule:

$$|f^{(4)}(x)| \leq M \;\;\;\;\; for \;\;\;\;\; a \leq x \leq b$$

$$\frac{b-a}{180}(M)(dx)^{4} = Error$$

3. The attempt at a solution

Ok so I have found the correct estimations using both methods easily, the trapezoidal approximation is: 1.5708 and Simpson's Rule is: 2.0944, those numbers check out in the back of the book, but when it comes to finding the error I think that it should be 0 because the max (M) is zero for both the second and fourth derivative but the book says otherwise. Heres what I did:

$$y=sin(x)$$

$$y'=cos(x)$$

$$y''=-sin(x)$$

$$y^{(3)}= -cos(x)$$

$$y^{(4)}= sin(x)$$

and Trapezoidal Rule using y'' is:

$$|y''( \pi)|=0$$

and

$$|y''(0)|=0$$

and that follows the same for $$y^{(4)}$$ so M is 0 and thus the entire equation is 0 and Error = 0 but the book states that the error for the trapezoidal approximation is:

$$\frac { \pi^{3}}{48} \;\;\;\;\; or \;\;\;\; .65$$

and the Error for Simpson's Rule is:

$$\frac { \pi^{5}}{2880} \;\;\;\; or \;\;\;\; .1$$

I don't see how they got this.... but I don't think the actual error rate is zero either because if it were then the trapezoidal and simpson approximation would be exactly equal, so where did I go wrong?

2. May 5, 2010

### Staff: Mentor

f''(x) = -sin(x) and f(4)(x) = sin(x). The maximum value of the absolute values of these functions is not zero. What you're looking for is the maximum value over the entire interval [0, pi].