fsolve返回函数不为零的点

Question

我是Python编程的新手，我正在努力优化一个简单的非线性函数。代码如下（我的计算机上有anaconda，包括numpy和scipy库）：

#!/usr/bin/python
# -*- coding: Latin-1 -*-

# System libraries

import os
import sys
import math
import getopt
import scipy

from scipy import optimize

#params
a = 78.0
b = 47.0
c = 0.0203242205945909

#initial guess (solution is not far to c so we use c parameter as initial guess)
guess = c   
#objective function to find root
def obj(_x, _a, _b, _c):
    f = (_c - (_x + (_b*x**_b)*(10**_a)))
    return f;

x = guess

sol = scipy.optimize.fsolve(lambda x: obj(x, _a=a, _b=b,  _c=c), guess)
#aaaand when we calculate the solution, it does not correspond to a root of the function!
print(sol)
print(obj(x, a, b, c))

使用脚本时的结果是：

[-1.38815731]
-1.40848153538

我希望：

[correct root that I don't know]
0.00000000000

为什么会给出结果？我做错了什么？如何使fsolve冗长？我在文档中没有这样做。