D

#include <bits/stdc++.h>

using namespace std;

#define ll     long long
#define _test   int _TEST; cin>>_TEST; while(_TEST--)
#define ff     first
#define ss     second
#define pb     push_back
#define ppb    pop_back

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

    _test
    {
        int n;
        double b, h;
        cin>>n>>b>>h;

        vector<ll int> y(n);
        for(auto &e: y)     cin>>e;

        double area = (b*h)/2;

        ll int prev = -1e9;
        double ans = 0;

        for(auto e: y)
        {
            if(prev+h > e)
            {
                ll int _h = prev+h - e;
                ll int _b = (_h*b)/h;
                ans -= (((_h*b))*_h);
            }

            ans += area*h*2.0;
            prev = e;
        }

        ans /= 2;
        ans /= h;

        cout<<fixed<<setprecision(10);
        cout<<ans<<"\n";
    }
}