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

  public class Guo04 { public static void main(String[] args) { double sum = 0, a = 2, b = 1, c = 0; for(double i = 1; i <= 20; i++) { sum = sum + a/b; c = a; a = a + b; b = c; }

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