C语言等差数列前n项合

#include<stdio.h>

int main()
{
    int i;
    int sum = 0;
    int n; 
    int d;
   
    printf ("输入等差数列的首项,公差以及所求项数(中间以空格为间隔):");
    scanf ("%d %d %d",&i,&d,&n);
   
    for (i ;i <= n; i+=d)//i=+2等价于让i+2;2就是这个等差数列的公差 
		sum = sum + i;
	
	printf ("该以%d为首项%d为公差的等差数列前%d项合为:%d\n",i,d,n,sum);
     
    return 0;
}