public class FractionAdd {  
public static void main(String[] args) {  
double sum = 0;  
double n1 = 1, n2 = 2, sn = 0;// 经分析，分子分母都为斐波那契数列  
for (int i = 1; i <= 20; i++) {// 数列前20项,循环20次  
            sum += n2 / n1;  
            sn = n1 + n2;  
            n1 = n2;  
            n2 = sn;  
        }  
        System.out.println("这个数列的前20项之和为：" + sum);  
    }  
}