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);
}
}
}
}