（二）向前 向后 中心差商

 

#coding=utf-8
from sympy import *

h = input("請輸入h的值:")
#定義變量x
x=Symbol("x")
#定義函數f
f = -0.1*x**4-0.15*x**3-0.5*x**2-0.25*x+1.2
d = diff(f,x,1)
#向前差商
d.subs(x,0.5)
g1 = (f.subs(x,0.5 + h) - f.subs(x,0.5))/ h
print "向前差商結果為："
print  g1
#向后差商
g2= (f.subs(x,0.5) - f.subs(x,0.5 - h)) / h
print "向后差商結果為："
print  g2
#中心差商
g3 = (f.subs(x,0.5 + h) - f.subs(x,0.5-h)) / (2 * h)
print "中心差商的結果為："
print  g3