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Homework Help: Basic logarithm

  1. Sep 10, 2010 #1
    I'm very new to logarithm.
    I've got homework to solve a ques using logarithm i.e multipication of 9356 by 0.396. And i've done it in following manner

    9356*0.396
    =log 9356 + log 0.396
    =3.9711-1.4023
    =2.5688

    I know i've done it in a wrong manner as the ans is wrong. I'm very new to it. I just know how to see logarithmic table and know basic formulae only. I just want to know the procedure to do such questions
     
  2. jcsd
  3. Sep 10, 2010 #2

    Mark44

    Staff: Mentor

    It should be obvious that you've done something wrong, since you have 9356 * .396 being equal to 2.5688.

    What you're really doing, but not showing, is taking the log of 9356 * .396.

    There's another error -- log(.396) is not -1.4023.
     
  4. Sep 10, 2010 #3
    Then what to do sir, this is my first ques of such a kind.
    I've to calculate using logarithm. Please tell me the first step, rest of all i think i'll do myself.
    Should i've to do log9356*log0.396?
     
  5. Sep 10, 2010 #4
    Hey this time i got right answer.
    I do it in this way
    I assume ans to b x then,
    Log(9356*0.396)=log x
    3.5688=log x
    x=antilog 3.5688= 3705
    And i thnk this is right
     
  6. Sep 10, 2010 #5

    Mark44

    Staff: Mentor

    You should have
    log(9356*0.396)
    = log 9356 + log 0.396
    Now continue from there.

    When you're done, you will have a number that is the logarithm of (9356*0.396).

    Presumably the problem wants you to find 9356*0.396, which is easy enough to do using multiplication (by hand or using a calculator), but more difficult if you have to use logs to get it.

    Let's call your answer L, where L = log(9356*0.396). To find (9356*0.396), exponentiate each side.
    L = log(9356*0.396)
    <==> 10L = 10log(9356*0.396)

    Hopefully, you know enough about the properties of logs to be able to simplify the right side.
     
  7. Sep 11, 2010 #6

    HallsofIvy

    User Avatar
    Science Advisor

    Are they still teaching the use of logarithms for problems like that? With calculators as common as they are now, I would think using logarithms for multiplications and divisions would be outmoded.

    (I am reminded of a science fiction story, I think by Isaac Asimov, taking place hundreds of years in the future, in which an engineer whips out his slide rule!)
     
  8. Sep 11, 2010 #7
    Not in electronics. Logarithms are still used extensively in electronics as dB. I use them nearly every day.

    A good example of doing multiplication and division by adding and subtracting logarithms.
     
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