（二）向前 向后 中心差商

 1 #coding=utf-8
 2 from sympy import *
 3 
 4 h = input("请输入h的值:")
 5 #定义变量x
 6 x=Symbol("x")
 7 #定义函数f
 8 f = -0.1*x**4-0.15*x**3-0.5*x**2-0.25*x+1.2
 9 d = diff(f,x,1)
10 #向前差商
11 d.subs(x,0.5)
12 g1 = (f.subs(x,0.5 + h) - f.subs(x,0.5))/ h
13 print "向前差商结果为："
14 print  g1
15 #向后差商
16 g2= (f.subs(x,0.5) - f.subs(x,0.5 - h)) / h
17 print "向后差商结果为："
18 print  g2
19 #中心差商
20 g3 = (f.subs(x,0.5 + h) - f.subs(x,0.5-h)) / (2 * h)
21 print "中心差商的结果为："
22 print  g3