import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }




void main() {
  try {
    for (; ; ) {
      const numCases = readInt;
      foreach (caseId; 0 .. numCases) {
        const N = readInt;
        const X = readReal;
        auto H = new real[N + 1];
        H[0] = 0.0L;
        foreach (i; 1 .. N + 1) {
          H[i] = readReal;
        }
        auto A = new real[N];
        foreach (i; 0 .. N) {
          A[i] = readReal;
        }
        
        auto Y = new real[N];
        foreach (i; 0 .. N) {
          Y[i] = H[i + 1] - H[i];
        }
        
        real ans = A[0] * X;
        foreach (k; 1 .. N) {
          bool ok = true;
          foreach (i; 0 .. k) {
            ok = ok && (A[i] > A[k]);
          }
          if (ok) {
            /*
              2 A[i] (x[i] / sqrt(x[i]^2 + Y[i]^2)) = 2 lam
              A[k] = lam
            */
            real cost = 0.0L;
            real sumX = 0.0L;
            foreach (i; 0 .. k) {
              const c = A[k] / A[i];
              const s = sqrt(1.0L - c^^2);
              const x = (c / s) * Y[i];
              cost += 2.0L * A[i] * sqrt(x^^2 + Y[i]^^2);
              sumX += x;
            }
            cost += A[k] * max(X - 2 * sumX, 0.0L);
            debug {
              writeln(A[0 .. k + 1], ": ", cost, " ", sumX);
            }
            chmin(ans, cost);
          }
        }
        writefln("%.12f", ans);
      }
    }
  } catch (EOFException e) {
  }
}