I was told by my teacher that the markers would expect you to use the series formulas since it's part of the topic. Using a formula (which is what general students would be using) does not test your knowledge at level the course is. However I was also told that the markers would mark you correct even if you did the wrong working out. Just check with your teachers first as you still need to do internal exams and therefore they can guide you on how to approach it if needed.