2/1，3/2，5/3，8/5，13/8，21/13...求出这个数列的前20项之和。

方法一：
#!/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：
#!/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：
#!/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)

以上实例输出结果为：

32.6602607986