Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Rounding to required level of accuracy

  1. Sep 28, 2011 #1
    1. The problem statement, all variables and given/known data

    Calculate each of the following to the required level of accuracy given that each number, other than those indicated in brackets, has been obtained by measurement.

    a) (3.142*1.95)/6*(3*5.44^2+1.95^2)
    (power 2, divisor 6, multiplier 3)

    b) (3.142*1.234)/12*( 0.424^2+0.424*0.951+0.951^2)
    (power 2 and divisor 12)

    2. Relevant equations



    3. The attempt at a solution

    a)My calculator gives me 94.5414368

    So I think 94.5 should be the answer since 1.95 has the least number of significant figures- three. My book, though, says it is 94.54

    b) The calculator gives 0.4805827337

    So my answer is 0.480 because 0.424 and 0.951 each have only 3 sig figs- least number of sig figures. But my book says the answer is 0.4806


    Please, show me where I am wrong.

    Thanks.
     
  2. jcsd
  3. Sep 28, 2011 #2

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    The rules for sig. fig.s are different for (addition&subtraction) than they are for (multiplication&division).

    Your answer (a) looks right to me. 1.95 is a factor of all the rest, so it should limit the result to 3 sig. fig.

    For (b) however, each factor in ( 0.4242+0.424*0.951+0.9512) is accurate to the 0.001 place, so the result of adding these three factors is also accurate to the 0.001 place. That give 4 sig. fig. for that result. Each of the other two factors, 3.142 & 1.234 also has 4 sig. fig.s.
     
  4. Sep 28, 2011 #3
    Thank you for the answer. I tried to apply the rule that used for (b) to another problem and got another wrong answer.

    [4.62^2-(7.16-2.35)]/ [2.63+1.89* √(73.24)]

    The answer is 0.8793. I thought the answer had to be rounded to 3 sig fig.
     
  5. Sep 28, 2011 #4

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Beats me!

    I agree with you on this one.

    Does your textbook have some unusual rules for these?
     
  6. Sep 28, 2011 #5

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Maybe the author treats "required level of accuracy" different than "significant figures".
    ???
     
  7. Sep 28, 2011 #6
    It's Engineering Math by Stroud. It's a great book. Sometimes unexpected problems with strange answers pop up. These are some of the few.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook