python例题及代码

1.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。

   a = 2
    b = 1
    s = 0
    for i in range(1,21):
        s += a/b
        a = a + b
        b = a - b
        if i == 20:
            print(s)

2题目：求1!+2!+3!+…+20!的和。

i = 1
s = 0
for o in range(1,21):
    i = i * o
    s = s + i
    if o == 20:
        print(s)

或者

s = 0
i = 1
o = 1
while o <= 20:
    i *= o
    o += 1
    s += i
print(s)

3.题目：有1、2、3、4个数字，能组成多少个互不相同且无重复数字的三位数？都是多少？

t = 0
for i in range(1,5):
    for o in range(1,5):
        for a in range(1,5):
            if i!=o and i!=a and o!=a:
                s = 100*i + 10*o + a
                print(s)
                t = t + 1
print(t)