#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(12);
  int tt;
  cin >> tt;
  while(tt--) {
    int n, x;
    cin >> n >> x;
    double x2 = x * 0.5;
    VL h(n + 1), a(n);
    rep(i, n) cin >> h[i + 1];
    rep(i, n) cin >> a[i];
    VL d(n);
    rep(i, n) d[i] = h[i + 1] - h[i];
    double ans = x * a[0];
    rep(i, n) if (i) {
      double tl = 0.0, tr = M_PI / 2;
      auto f = [&](double t) {
        double res = 0;
        double p = 0;
        rep(j, i) {
          p += d[j] / tan(t);
          res += a[j] * d[j] / sin(t);
          if (j + 1 < i) {
            double tmp = cos(t) * a[j] / a[j+1];
            if (tmp > 1.0) return 1e20;
            t = acos(tmp);
          }
        }
        if (p > x2) return 1e20;
        res += a[i] * (x2 - p);
        return res;
      };
      while(true) {
        double tc1 = (2 * tl + tr) / 3;
        double tc2 = (tl + 2 * tr) / 3;
        if (!(tl < tc1 && tc1 < tc2 && tc2 < tr)) break;
        double d1 = f(tc1);
        double d2 = f(tc2);
        if (d1 < d2) tr = tc2;
        else tl = tc1;
      }
      chmin(ans, 2 * f(tl));
    }
    cout << ans << '\n';
  }
}