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 August 31st, 2015, 04:08 AM #1 Newbie   Joined: Aug 2014 From: Australia Posts: 2 Thanks: 0 Calculating using logarithms Hi all, I have tried several methods for calculating a value using a formula with logarithms and I get varying answers depending on how I calculate it. I am leaning towards calculating the formula each time and subtracting the difference. This is the formula: $\log y = 0.8 - 4.1 \times {10^{ - 4}}{\left( {8 + \log (x)} \right)^3}$ Now I need to find the difference between x=15 and x=320, so I have tried: ${10^{\log \left( {\frac{{0.8 - 4.1 \times {{10}^{ - 4}}{{\left( {8 + \log (15)} \right)}^3}}}{{0.8 - 4.1 \times {{10}^{ - 4}}{{\left( {8 + \log (320)} \right)}^3}}}} \right)}}$ and: ${10^{((0.8 - 4.1 \times {{10}^{ - 4}}{{\left( {8 + \log (15)} \right)}^3}) - (0.8 - 4.1 \times {{10}^{ - 4}}{{\left( {8 + \log (320)} \right)}^3}))}}$ and: ${10^{(0.8 - 4.1 \times {{10}^{ - 4}}{{\left( {8 + \log (15)} \right)}^3})}} - {10^{(0.8 - 4.1 \times {{10}^{ - 4}}{{\left( {8 + \log (320)} \right)}^3})}}$ Each time I get a different answer. I'm guessing the last method is the right one, but why do the others produce different results? Thanks in advance. August 31st, 2015, 04:45 AM #2 Newbie   Joined: Aug 2014 From: Australia Posts: 2 Thanks: 0 Apologies, I see why now... When adding and subtracting they need to be evaluated beforehand. Tags calculating, logarithms Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post Shawn Algebra 4 May 23rd, 2013 11:50 AM BrianMX34 Algebra 7 September 6th, 2012 01:32 PM OriaG Algebra 5 August 2nd, 2012 12:59 AM yyttr4 Algebra 1 June 21st, 2010 07:57 PM Cstolworthy Algebra 1 October 15th, 2008 02:25 PM

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