计算前20项和

//author：yangyang#include "stdafx.h"#define N 20//定义为求20项的和double sum(double sum);int main(){    double res = 0;    for (int i = 0; i < N; i++)    {        res = sum(res);//运用递归，求和    }    printf("%f", res);    return 0;}//递归求和函数static double a = 1.0, b = 2.0;double sum(double sum){    sum +=  b / a;    double c = b;    b = a + b;    a = c;    return sum;}