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Homework Help: Questions about Sig Figs

  1. Feb 20, 2005 #1
    I understand that if your doing multiplication, round to the least amount of sig figs. Addition: Round to the least amount of decimal places.

    What about a question with both multiplication, division and addition, subtraction?

    Such as

    (5.2 x 10^4 + 4.2 x 10^-2)/ 3.6 x 10^2

    How would I apply sig figs on that?

    Thx in advance
  2. jcsd
  3. Feb 20, 2005 #2
    [tex]\frac{52000+0.042}{360}=\frac{52000.042}{360}\approx 144.4445611\approx 144[/tex]

    That is how I would do it. 3 significant digits in the 360, so the same for the answer, and don't round anything until the end.
  4. Feb 20, 2005 #3
    I think it's about context too. If the question was about a length measurement taken (just say) and the value was 360mm, then shouldn't that should be taken to 2 sig.fig (unless they actually say it was measured to 3 sigfig)?
  5. Feb 20, 2005 #4
    I'm assuming the instrument used had millimeter markings. Good point, though.
  6. Feb 20, 2005 #5
    thx guys :D

    I think 360 is 2 sig figs

    360.0 = 4 sig figs
  7. Feb 20, 2005 #6

    Doc Al

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    Staff: Mentor

    When written as in your first post (3.6 x 10^2) it has 2 sig figs for sure; 3 sig figs would be written as 3.60 x 10^2
  8. Feb 20, 2005 #7
    Oh boy, big mistake on my part. Corrected:

    [tex]\frac{52000+0.042}{360}=\frac{52000.042}{360}\approx 144.4445611\approx 1.4\times{10^{2}}[/tex]

    Firstwave, do the calculation, then look over to see the least number of significant digits in the original data. Sorry for misleading you earlier; my mistake.
  9. Feb 20, 2005 #8
    ok thx all
  10. Feb 21, 2005 #9


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    Homework Helper

    Sirius' solution is correct, but the pedantic side of me insists I point out that the numerator of that expression has only two significant digits, despite being written as though it had eight. Since you don't really know what the digits are in the "52000" after the 2, adding the .042 makes no significant difference in the number.
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