def solve():
    n, x = map(int, input().split())
    H = list(map(int, input().split()))
    A = list(map(int, input().split()))
    ans = x * A[0]
    z = x / 2
    h = H[0]
    a = (h * h + 1) ** 0.5 * A[0]
    for h2, b in zip(H[1:], A[1:]):
        f = lambda x:((h * h + x * x) / (h * h + 1)) ** 0.5 * a + (z - x) * b
        l = 0
        r = z
        for _ in range(100):
            llr = (2 * l + r) / 3
            lrr = (l + 2 * r) / 3
            if f(llr) < f(lrr):
                r = lrr
            else:
                l = llr
        tmp = 2 * min(f(l), f(llr), f(lrr), f(r))
        ans = min(ans, tmp)
        f = lambda x:((h * h + x * x) / (h * h + 1)) ** 0.5 * a + (h2 * h2 + (1 - x) ** 2) ** 0.5 * b
        l = 0
        r = 1
        for _ in range(100):
            llr = (2 * l + r) / 3
            lrr = (l + 2 * r) / 3
            if f(llr) < f(lrr):
                r = lrr
            else:
                l = llr
        tmp = min(f(l), f(llr), f(lrr), f(r))
        a = tmp
        h += h2


    print(ans)

        


for _ in range(int(input())):
    solve()