for _ in range(int(input())):
    int(input())
    w=[float(x)for x in input().split(" ")]
    x,y=w[0::2],w[1::2]
    print(abs(sum([x[i%len(x)]*(y[(i-1)%len(y)]-y[(i+1)%len(y)])for i in range(len(x))]))/2)