# lecture 10 (Starting Numerical Integration, Trapezoid Rule, and Simpson's Rule)

## Analytic vs Numerical Method

In calculus, you do something like:

In Numerical, we start with the Lagrange Interpolant, and do some other stuff:

Actually, it’s pretty simple, we essentially just transform the function into a polynomial using the Lagrange Interpolant, and then we can differentiate that polynomial. It’s really easy to differentiate polynomials, so that’s a pretty solid method.

So for our differentiation, we simply have:

$$

f’(x)=P’(x) + error’ \leftarrow \textrm{Lagrange Interpolant: }f(x) = P(x) + error \rightarrow ???? f’(x)=P’(x) + error’ \leftarrow \textrm{Lagrange Interpolant: }f(x) = P(x) + error \rightarrow \int_a^bf(x)dx=\int_a^bP(x)dx+\int_a^b\textrm{error }dx $$

## Numerical Integration

Today we’ll be looking at the (n+1) point formula again, but for integration. This is called the Newton’s-Cotes formula, which converges as a rate of \(O(h^{2n+1})\).

Suppose our goal is to compute

Graphically, our goal is to calculate the area under the curve between a and b. If we look at this for n = 1, i.e. using only two points to approximate, you can see we get a trapezoid! This is known as the **Trapezoid Rule**.

The area of this trapezoid is simply \(\frac{f(a)+f(b)}{2}(b-a)\), so it’s pretty easy to calculate. Wowza. You can already do numerical integration. But it’s not very accurate. How inaccurate? Let’s find out!

Back to the Lagrange Interpolant:

Now we just have to integrate it from \(x_0\) to \(x_1\). To make that more obvious, let’s expand the Lagrange Interpolants

Finally let’s replace \(x_1 - x_0\) with h

\(f(x_n)\) and \(h\) are both constants, so we can pull them out of the integral we’re about to do:

If we solve that integral:

If we plug in the x’s, and then simplify, we get something like:

If you’re having trouble doing that simplification, don’t forget that \(x_1-x_0 = h\). That might help.

Now we just have that pesky error term remaining.

The \(f’’\) term is definitely the most ugly part of that integral, so let’s just bring it out. We can do this because \(\xi\) is bound by the \([x_0,x_1]\) interval, so we can use some Mean Value Theorem magic to just bring it out: We’ll end up with:

as our error. Remember that was all for n = 1, aka using two points, aka the Trapezoid rule. If we want to get a little bit fancier, we can use **Simpson’s rule**, which is using n = 2, aka 3 points. Now, instead of having a straight line to cross the gap between \(a,f(a)\) and \(b,f(b)\), we’ll make a quadratic polynomial using the three points. You can solve for the error in Simpson’s Rule in the same way as we did for the Trapezoid Rule, but the integration is a lot uglier, because there’s so many points. It’s not really particularly difficult, and is a great exercise to do yourself, but, it is rather long. Anyway, here’s the final result of those mathematics. Note the error here converges at order 5 when using three points (n = 2), as we mentioned at the very beginning, these formulas converage at a rate of \(O(h^{2n+1})\).

#### Example

using

- Exact
- Trapezoid
- Simpson’s
Exact is pretty simple, just integrate to get

For Trapezoid, we just plug in to the trapezoid formula from above:

For Simpson’s Rule, we plug in again:

The take-away? Simpson’s Rule is exact for a polynomial of degree three. Also, it’s generally more accurate than the trapezoid rule, without being terribly more difficult. These are all called**Newton’s-Cotes**methods. In Newton’s-Cotes, you can have open or closed.- Closed Newton’s-Cotes: Take endpoints into account
- Open Newton’s-Cotes: No endpoints (aka midpoint)