数列递推，求出公式计算按要求输出即可。

代码 ：

import java.util.*;

public class Main10014 {

	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int t = scan.nextInt();
		while(t-- > 0) {
			int n = scan.nextInt();
			double a0 = scan.nextDouble();
			double an = scan.nextDouble();
			double c, sum = 0;
			for(int i=0; i<n; i++) {
				c = scan.nextDouble();
				sum += (n-i)*c;
			}
			
			System.out.printf("%.2f\n",((n * a0 + an - 2 * sum) / (n + 1)));
			if(t != 0)
				System.out.println();
			
		}

	}

}

uva 10014 - Simple calculations