水题堆3.A - A1 = ?

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int n,i;
    double a0,an,s,k,c[3010],a1;
    while(scanf("%d",&n)!=EOF){
        scanf("%lf%lf",&a0,&an);
        for(i=1;i<=n;i++)scanf("%lf",&c[i]);
        k=2*n;
        s=0;
        for(i=1;i<=n;i++){
            s+=k*c[i];
            k=k-2;
        }
        a1=(an-s+n*a0)/(n+1);
        printf("%.2lf\n",a1);
    }
    return 0;
}

找规律，写了三项

A2=2A1+2C1-A0;

A3=3A1+4C1+2C2-2A0;

A4=4A1+6C1+4C2+2C3-3A0;

找出规律

AN=NA1+(2N-2)C1+(2N-4)C2+...+2CN-1-(N-1)A0；