Secant Method
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import numpy as np
import matplotlib.pyplot as pt
Here's a function: (Look here! No derivative.)
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def f(x):
return x**3 - x +1
xmesh = np.linspace(-2, 2, 100)
pt.ylim([-3, 10])
pt.grid()
pt.plot(xmesh, f(xmesh))
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guesses = [2, 1.5]
Evaluate this cell many times in-place (using Ctrl-Enter)
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# grab last two guesses
x = guesses[-1]
xbefore = guesses[-2]
slope = (f(x)-f(xbefore))/(x-xbefore)
# plot approximate function
pt.plot(xmesh, f(xmesh))
pt.plot(xmesh, f(x) + slope*(xmesh-x))
pt.plot(x, f(x), "o")
pt.plot(xbefore, f(xbefore), "o")
pt.ylim([-3, 10])
pt.axhline(0, color="black")
# Compute approximate root
xnew = x - f(x) / slope
guesses.append(xnew)
print(xnew)
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