2.分别用while和for循环编程，实现求一下数列前20项之和 2/1, 3/2, 5/3, 8/5, 13/8, ……

以下是使用while和for循环分别实现求数列前20项之和的C语言代码：

1.使用while循环：

#include <stdio.h>

int main() {
    double sum = 0, a = 2, b = 1, c, term;
    int i = 1;
    while (i <= 20) {
        term = a / b;
        sum = sum + term;
        c = a + b;
        b = a;
        a = c;
        i++;
    }
    printf("The sum is: %.2f\n", sum);
    return 0;
}

2.使用for循环：

#include <stdio.h>

int main() {
    double sum = 0, a = 2, b = 1, c, term;
    int i;
    for (i = 1; i <= 20; i++) {
        term = a / b;
        sum = sum + term;
        c = a + b;
        b = a;
        a = c;
    }
    printf("The sum is: %.2f\n", sum);
    return 0;
}