public void TestSurface()
        {
            sphere.Radius = 3;
            Assert.Equal(113.10, sphere.Surface(), 2);

            sphere.Radius = 77;
            Assert.Equal(74506.01, sphere.Surface(), 2);

            sphere.Radius = 6453;
            Assert.Equal(523278865.12, sphere.Surface(), 2);

            sphere.Radius = 1;
            Assert.Equal(12.57, sphere.Surface(), 2);
        }