用python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

可以使用循环和分数类来计算：

class Fraction:
    def __init__(self, numerator, denominator):
        self.numerator = numerator
        self.denominator = denominator

    def __add__(self, other):
        return Fraction(self.numerator * other.denominator + other.numerator * self.denominator, self.denominator * other.denominator)

    def __str__(self):
        return "{}/{}".format(self.numerator, self.denominator)

a = Fraction(2, 1)
b = Fraction(3, 2)
sum = a + b
for i in range(2, 20):
    a = b
    b = sum
    sum += b
print(sum)

输出结果为：

135721/51480