1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Logarithms and scientific calculator

  1. Nov 2, 2014 #1
    1. The problem statement, all variables and given/known data
    y= -2log3(x-3) -1

    2. Relevant equations
    I am using a SHARP EL-546W scientific calculator, and I do not know what steps to take in order to find a point given an x value. i.e. if x=3, then y=6. I cannot seem to get 6 on my own and I have tried a wide variety of methods and button sequences.

    3. The attempt at a solution
    Please instruct me on how to find logs on my calculator, thank you
     
  2. jcsd
  3. Nov 2, 2014 #2

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    I cannot figure out what is your expression. Do you mean ##y = -2 \log_{3}(x-3) -1## or ##y = -2 (\log 3)(x-3) -1## or ##y = -2 \log(3(x-3)) - 1##. If you mean one of the last two what "base" are you using for the logarithm? Base 10? Base ##e##? You must use parentheses when typing formulas, tp make your meaning clear.

    Anyway, you will NEVER get y = 6 when you put x = 3. If you mean either the first or third form above, the logarithm does not exist when x = 3 because it would be log(0)---and there is no such thing---while if you mean the second one you would get y = -1 when x = 3.
     
  4. Nov 2, 2014 #3
    sorry, it is the first expression you mentioned
     
  5. Nov 2, 2014 #4

    Mark44

    Staff: Mentor

    If x = 3, then x - 3 = 0, so log3(x - 3) is undefined.
     
  6. Nov 2, 2014 #5

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    So, you want log to base 3. I am not familiar with your calculator, but most calculators do not have buttons for logs to arbitrary bases, but almost always allow you to choose between base 10 and base ##e##. You can get ##\log_3 w## either in terms of ##\log_{10} w## or ##\log_{e} w \equiv \ln w##. In fact, if you have two bases ##a## and ##b##, you can get ##\log_{a} w## in terms of ##\log_b w##:
    [tex] \log_a w = \frac{\log_b w}{\log_b a},\\
    \text{so }\\
    \log_3 w = \frac{\log_{10} w}{\log_{10} 3}\\
    \text{or}\\
    \log_3 w = \frac{\ln w}{\ln 3} [/tex]
     
  7. Nov 2, 2014 #6
    ok thank you for the help everything is clear now
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Logarithms and scientific calculator
  1. Logarithm's and Such (Replies: 4)

  2. Logarithm's and Such (Replies: 12)

Loading...