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#include<stdio.h>
//求前n项和 
int main()
{int shouxiang,qianjixiang,he;
shouxiang=1; 
he=0;
for (qianjixiang=1;qianjixiang<=3;qianjixiang++)
    {he=shouxiang+he;
    shouxiang=shouxiang*2; } 
    printf("%d",he);
 }