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Sig. Fig. question.

  1. Sep 7, 2014 #1
    1. The problem statement, all variables and given/known data
    Calculate (.82+.042)(4.4*10^3) keeping only significant figures.


    3. The attempt at a solution
    I got .82+.042=.86
    (.86)(4.4*10^3)=3784
    So sig. fig says the answer should be 3800. Please explain what I did wrong. Thank you.
     
  2. jcsd
  3. Sep 7, 2014 #2

    BvU

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    Clue is in the number of significant figures they give you in the numbers provided. And they did a good job writing e.g. 4.4*103 instead of 44000 to make clear the number of significant digits....

    And you do get the correct idea writing .86 and not .862 !
     
  4. Sep 7, 2014 #3
    Clue is in the number of significant figures they give you in the numbers provided. And they did a good job writing e.g. 4.4*103 instead of 44000 to make clear the number of significant digits....

    And you do get the correct idea writing .86 and not .862 !

    So .82+.042=.86 according to sig. figs., then (.86)(4.4*10^3) would equal 3.8*10^3 since .86 has 2 sig. figs. and 4.4*10^3 has 2 sig. figs.?


    My professor has a program called drillmaster and according to this program the answer is 3792
     
    Last edited: Sep 7, 2014
  5. Sep 7, 2014 #4

    BvU

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    The program can't understand what is asked in the exercise. It can only process the given numbers. At that, it does a pretty bad job too: at least it should not truncate 3792.8 to 3792 but round off instead.

    Almost. You don't really want to do the rounding off at every intermediate step. 82 42 and 44 are 2 siginificant figures, so from the calculated result 3792.8 you want to round off to two as well: 3.8 10^3.

    Note that generally one provides an extra digit if the first is a 1. So 98 + 3.2 is not 100 but 101.
    (or rather 10.1 * 10^1).
     
  6. Sep 7, 2014 #5
    The program is not like a calculator. It gives the question then a list of answers and you select one. I can try to post a pic.
     

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  7. Sep 7, 2014 #6

    HallsofIvy

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    Each of the numbers given has two significant figures so the answer must have two significant figures. Only one given answer has two significant figures- that is a: 3800. Are you saying that you checked (a) and were told it was wrong? If so, discuss this with your teacher.
     
  8. Sep 7, 2014 #7
    That is what I am saying. The program says the correct answer is c.
     
    Last edited: Sep 7, 2014
  9. Sep 8, 2014 #8

    Borek

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    So the program is wrong. Note, that it almost for sure uses predefined questions (that is, it doesn't calculate the answer, it has it on file; whoever entered the answer into the file, made a mistake).
     
  10. Sep 8, 2014 #9
    Thank you everyone.
     
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