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();


			for (int i = 0; i < n; i++) {
				System.out.println();
				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);

			}

		}
	}
}