Slight difference in calculation?

I'm not sure how answers are marked in A level Mathematics exams (if it counts, I'm sitting for the CIE board) but when I am doing practice questions based on a book, sometimes my answer may differ by a few hundredths or tenths (usually the third significant figure). When this happens, I usually assume that my calculation is correct and that the difference simply happens because of a choice I use to use or not use the correct form, not the one rounded off to 3 s.f. Would my marks be affected by these differences in an exam and if they do, when do I know whether I should use the rounded off figure or the exact form?

For instance in the work I've recently done, the question comes in 2 parts, the first is that I need to find the area of the sector and the second is the area of the chord of said sector. What I did was subtract the area of triangle from my previous answer but my answer was off by 2 hundredth. I did the calculation again using the correct form of the sector, not the one rounded to 3 s.f. and got the answer written in the answer booklet.

Would my answer be considered correct?

Answers
Hi,After a calculation, you get the "exact value" to your calculation.When you write your answer, you will always write the exact value unless asked to round or give to a certain number of significant figures.So if you write your answer exactly when it wasn't specified to round it, then you will be correct.However, in two part questions is where I believe you are getting the wrong answers.I see many students who take their answer to the first part of a question, and use that rounded value in their calculations for part two of the question.Never use a rounded value in any of your calculations unless asked to.For example, For the first part of your question, you got the area of the sector as, say, 10.45.And you gave the answer as 10.5 because part one asked you to round your answer to one decimal place.Now, for part two, when you use the area of the sector, you need to use the 10.45 value, not the rounded 10.5 value in your calculation.You can then round your part two calculation afterwards if it asks.I hope that helps. 
Myles W.
24 August 2018
Yes, it does, thank you very much. The answer provided in my book is always correct to 3 significant figures unless specified otherwise so I've been following that as a guide.
droplette
25 August 2018
Best thing to do is to not round any number before the end of the question, only the final answer. It's not a safe thing to do as you're introducing a discrepancy from the exact answer early in the question which might be multiplied by a large number later in the question and that error will be multiplied as well so your final answer will be too far off.To answer whether that kind of answer on an exam would be accepted or not depends on the marking scheme. Usually questions involving PI, like the one you had have some error tolerance but don't count on that. In this case - textbook answers clearly show you were too far off and you would lose the mark. Easiest solution - don't round anything until the final answer.Hope that helpsMarcin
Marcin B.
25 August 2018
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