题目：有一分数序列：2/1，3/2，5/3，8/5，13/8，21/13...求出这个数列的前20项之和。
程序分析：请抓住分子与分母的变化规律。
程序源代码：
方法一：
实例
#!/usr/bin/python
# -*- coding: UTF-8 -*-
a = 2.0
b = 1.0
s = 0
for n in range(1,21):
s += a / b
t = a
a = a + b
b = t
print (s)
方法二：
实例
#!/usr/bin/python
# -*- coding: UTF-8 -*-
a = 2.0
b = 1.0
s = 0.0
for n in range(1,21):
s += a / b
b,a = a , a + b
print (s)
s = 0.0
for n in range(1,21):
s += a / b
b,a = a , a + b
print (s)
方法三：
实例(Python 2.x)
#!/usr/bin/python
# -*- coding: UTF-8 -*-
a = 2.0
b = 1.0
l = []
l.append(a / b)
for n in range(1,20):
b,a = a,a + b
l.append(a / b)
print reduce(lambda x,y: x + y,l)
实例(Python 3.x)
#!/usr/bin/python
from functools import reduce
a = 2.0
b = 1.0
l = []
l.append(a / b)
for n in range(1,20):
b,a = a,a + b
l.append(a / b)
print (reduce(lambda x,y: x + y,l))
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