code: def test_to_joint_gaussian(self):
      self.model.add_cpds(self.cpd1, self.cpd2, self.cpd3)
      jgd = self.model.to_joint_gaussian()
      self.assertEqual(jgd.variables, ["x1", "x2", "x3"])
      np_test.assert_array_equal(
          jgd.mean, np.array([[1.0], [-4.5], [8.5]])
      )
      np_test.assert_array_equal(
          jgd.covariance,
          np.array(
              [[4.0, 2.0, -2.0], [2.0, 5.0, -5.0], [-2.0, -5.0, 8.0]]
          ),
      )
  
file_path: pgmpy/pgmpy/tests/test_models/test_LinearGaussianBayesianNetwork.py
imports: import unittest
  import numpy
  import numpy.testing
  from pgmpy.factors.continuous import LinearGaussianCPD
  from pgmpy.factors.discrete import TabularCPD
  from pgmpy.models import LinearGaussianBayesianNetwork
label_desc: Use image node_id 4 for calling the TestLGBNMethods obj's underlying member method code with example usage: obj.test_to_joint_gaussian() without return types