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Homework Help: Adding and Subtracting Significant Figures

  1. Oct 4, 2016 #1
    1. The problem statement, all variables and given/known data

    To add or subtract sig figs, it is to my understanding that the answer is reported so that it reflects the least precise number.

    For the example 43.4 - 22 - 3, would I report my answer as 18 or 20?
    Thanks. Any help is very much appreciated!

    2. Relevant equations

    3. The attempt at a solution
    The least precise number is 3 but I am not sure if I would report my answer with 1 or 2 sig figs. I am guessing that my answer should be 20 since it has only one significant figure as well.
  2. jcsd
  3. Oct 5, 2016 #2


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    Gold Member

    Been a long time since I did such a thing formally but as I recall the normal practice is to say that your error is the RMS of the individual errors, which in this case would be sqrt((.05*2 + .5*2 + .5*2)/3) = .7 so your answer is 18.4 +/- .7
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