有一分数序列 2 / 1,3 / 2,5 / 3,8 / 5,1

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 double sum = 0;
            double a = 2;
            double b = 1;
            for (int i = 1; i < 21; i++)
            {
                sum += a / b;
                double temp = a;
                a = a + b;
                b = temp;
            }
            Console.WriteLine(sum);