public static void testValue() {
		Parabola p = new Parabola (1, -4, 3);
		double y = p.value(1.0);
		assert y == 0.0;
		y = p.value(2.0);
		assert y == -1.0;
		y = p.value (7.0);
		assert y == 24.0;
		y = p.value (-2.5);
		assert y == 19.25;
		System.out.println("value OK.");
		}