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Math Help - Rounding numbers

  1. #1
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    Question Rounding numbers

    Hi everyone! I have a question about rounding numbers:

    When using rounding arithmetic, do I have to round the number after I complete the whole operation or do I have to round them during every step? For example: I was asked to get an approximation of one of the roots of (1/3)x^2-(123/4)x+1/6=0, using the formula x1=(-2c)/(b+sqrt(b^2-4ac)) with 4-digit rounding arithmetic. Do I have to substitute directly the values of a, b and c and round the number I get or do I have to use the rounded values in the formula (e.g. 0.1667 instead of 1/6)?

    Someone told me that I have to do it using the rounded values in the formula, however I got a 0 on the denominator. If someone could show me step by step how to do this exercise I'd be very grateful.

    Edit.
    Nevermind. I found out that I can't use that formula because the value of b is negative and I instead I must use the general quadratic formula to get the approximation.
    Last edited by Cesc1; February 3rd 2013 at 12:36 PM. Reason: Solved
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  2. #2
    Super Member ILikeSerena's Avatar
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    Re: Rounding numbers

    Hi Cesc1!

    As a rule, it's best not to round in intermediate steps at all.
    Let your calculator do the work.
    If you have to (because you're starting to lose track), you can round, but always keep a couple of extra digits when you do.

    Apparently you are doing an exercise to train the latter option.
    IMO, to get the best result, you should still round as little as possible.
    But it seems you are supposed to round a couple of intermediate results to 4 digits.
    You can pick yourself what you want to round.

    Btw, your formula appears to contain a typo.
    I think it should be: x1=(-2a)/(b+sqrt(b^2-4ac)).
    Thanks from Cesc1
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  3. #3
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    Re: Rounding numbers

    Quote Originally Posted by ILikeSerena View Post
    Hi Cesc1!

    As a rule, it's best not to round in intermediate steps at all.
    Let your calculator do the work.
    If you have to (because you're starting to lose track), you can round, but always keep a couple of extra digits when you do.

    Apparently you are doing an exercise to train the latter option.
    IMO, to get the best result, you should still round as little as possible.
    But it seems you are supposed to round a couple of intermediate results to 4 digits.
    You can pick yourself what you want to round.

    Btw, your formula appears to contain a typo.
    I think it should be: x1=(-2a)/(b+sqrt(b^2-4ac)).
    Yeah, I practically never round numbers but they asked me to do it for my numerical analysis homework, so I was having a little problem.

    Anyway, thanks for your help!
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