import java.util.Scanner;

public class SimpleCalc {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);

        while (in.hasNext()) {
            int n = in.nextInt();

            boolean cp = false;
            for (int i = 0; i < n; i++) {
                if (cp == true)
                    System.out.println();
                cp = true;
                int x = in.nextInt();
                double a0 = in.nextDouble();
                double af = in.nextDouble();

                double a1 = 0;

                double c = 0;
                double k = 0;
                for (int j = 0; j < x; j++) {
                    c = in.nextDouble();
                    c = (double) (-1)
                            * ((double) 2 * (double) (x - j) * (double) c );

                    k = (double) c + (double) k;

                }
                k=(double)k/(double)(x+1);
                a1 = ((double) (x * a0) + af) / (double) (x + 1) + (double) k;
                System.out.printf("%.2f\n", a1);

            }

        }
    }
}