public void TestSubtract()
{
Cartesian original = new Cartesian(10.0, 20.0, 30.0);
Cartesian toSubtract = new Cartesian(1.0, 2.0, 3.0);
Cartesian result = original - toSubtract;
Assert.AreEqual(9.0, result.X);
Assert.AreEqual(18.0, result.Y);
Assert.AreEqual(27.0, result.Z);
result = original.Subtract(toSubtract);
Assert.AreEqual(9.0, result.X);
Assert.AreEqual(18.0, result.Y);
Assert.AreEqual(27.0, result.Z);
}