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Is there a simple formula when for getting the right sig-digs?

  1. Sep 23, 2012 #1
    1. The problem statement, all variables and given/known data

    I have no problem knowing how to find the right sig-digs when just doing one of: adding, subtracting, multiplying or dividing. But, I know doing them all at once in more complicated equations gets trickier.

    For example, something like 110.0 + (500434.0 - 34.0)/0.37

    2. Relevant equations



    3. The attempt at a solution

    This is my thought process for finding sig-digs when addition/subtraction is combined with multiplication/division. The answer may be correct by luck, but I know that this is not the right way to come up with the correct number of sig-digs.

    110.0 + (500434.0 - 34.0)/0.37
    = 110.0 + 500434.0/0.37
    = 110.0 + 1.4*10^6 (I make 500434/0.37 equal 2 sig digs because the denominator only has 2 sig-digs.)
    = 1.4001100*10^6 (Then I increase the sig-digs by 5 because I am adding 110.0.)

    I know this process is wrong because my professor tried to explain to me that there is an intermediate step that I am missing. I can't grasp what it is though.

    If it is correct, it is because of pure luck.
     
    Last edited: Sep 23, 2012
  2. jcsd
  3. Sep 23, 2012 #2

    vela

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    You made a sign error in the very first step. You should fix that first.
     
  4. Sep 23, 2012 #3
    Thanks, I meant to put 5400 - 34 instead of 34 - 5400, so I can use both + and - in the example.
     
  5. Sep 23, 2012 #4

    vela

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    How did you go from ##110 + 1.3437\times10^5## to ##111.3437\times10^5##? Remember PEMDAS.
     
  6. Sep 23, 2012 #5
    I don't know why I am so careless today (it's very frustrating being in university and not knowing this yet.).

    I changed the question too.
     
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