Sn的公式求和

解题思路:S0 = 0; A0 = 0

A1 = A0 * 10 + 2 = 2;            S1 = A1 + S0 = 2 ;

A2 = A1 * 10 + 2 = 22;          S2 = A2 + S1= 22 + 2;

A3 = A2 * 10 + 2 = 222;        S3 = A3 + S2 = 222 + 22 + 2;

.........以此类推

An = A(n-1) * 10 + 2;    Sn = An + S(n-1);
注意事项:

可能要注意的就是循环这里，如果判断条件是在大于或小于的条件上还加上等于，初始i应赋值1，否则赋值0；

参考代码:

#include<stdio.h>

int main(void)

{

    int An = 2, Sn = 0;  //此时就相当于 A1 = 2, S0 = 0;

    int n, i;

    scanf("%d", &n);

    for (i = 0; n > i; i++)

    {

        Sn = An + Sn;

        An = An * 10 + 2;

    }

    printf("%d", Sn);

    return 0;

}