@Test
    public void testAdditionNum() {
        String input = "((2.0 + 3.5) + ((x * 2) * 3.5))";
        String var = "x";
        String expected = "((0 + 0) + (((x * 2) * 0) + (3.5 * ((x * 0) + (2 * 1)))))";
        Differentiator diff = new Differentiator();
        assertEquals(expected, diff.evaluate(input, var));
    }