# Trapezoid

In [1]:

```
# coding: utf-8
# # Basic Numerical Integration: the Trapezoid Rule
# A simple illustration of the trapezoid rule for definite integration:
#
# $$
# $$
# <br>
# First, we define a simple function and sample it between 0 and 10 at 200 points
# In[1]:
get_ipython().magic('matplotlib inline')
import numpy as np
import matplotlib.pyplot as plt
# In[2]:
def f(x):
return (x-3)*(x-5)*(x-7)+85
x = np.linspace(0, 10, 200)
y = f(x)
# Choose a region to integrate over and take only a few points in that region
# In[3]:
a, b = 1, 8 # the left and right boundaries
N = 5 # the number of points
xint = np.linspace(a, b, N)
yint = f(xint)
# Plot both the function and the area below it in the trapezoid approximation
# In[4]:
plt.plot(x, y, lw=2)
plt.axis([0, 9, 0, 140])
plt.fill_between(xint, 0, yint, facecolor='gray', alpha=0.4)
plt.text(0.5 * (a + b), 30,r"$\int_a^b f(x)dx$", horizontalalignment='center', fontsize=20);
# Compute the integral both at high accuracy and with the trapezoid approximation
# In[5]:
from __future__ import print_function
from scipy.integrate import quad
integral, error = quad(f, a, b)
integral_trapezoid = sum( (xint[1:] - xint[:-1]) * (yint[1:] + yint[:-1]) ) / 2
print("The integral is:", integral, "+/-", error)
print("The trapezoid approximation with", len(xint), "points is:", integral_trapezoid)
```

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