@Test
 public void geometricMeanTwoNegativeOperandsExpression() {
   double result = Calculator.getResult(-2, -8, Calculator.OPERATOR.GEOMETRIC_MEAN);
   Assert.assertTrue(Double.isNaN(result));
 }
 @Test
 public void minusExpression() {
   double result = Calculator.getResult(2, 2, Calculator.OPERATOR.MINUS);
   Assert.assertEquals(0.0, result, 0.0);
 }