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August 31st, 2015, 05:08 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, 05:45 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.


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