CrazCo Posted January 17, 2009 Share Posted January 17, 2009 Example 5500 = 25500 * (0.85)^x Link to comment Share on other sites More sharing options...
mooeypoo Posted January 17, 2009 Share Posted January 17, 2009 Do you know about logarithms? I don't want to expose the answer straight-out, but you might want to read a bit about them http://en.wikipedia.org/wiki/Logarithm and http://mathworld.wolfram.com/Logarithm.html are good sources to get you started. Link to comment Share on other sites More sharing options...
CrazCo Posted January 17, 2009 Author Share Posted January 17, 2009 interesting sites. 5500 divid. 25500 = 0.21 so log 0.21 divided by log 0.85 = x Link to comment Share on other sites More sharing options...
mooeypoo Posted January 17, 2009 Share Posted January 17, 2009 What is the base of your logs? Remember, [math]x = b^y[/math] means that [math]y = \log _b{x}[/math] and check this out: http://en.wikipedia.org/wiki/Logarithm#Change_of_base but you're definitely on the right track! Link to comment Share on other sites More sharing options...
Cap'n Refsmmat Posted January 17, 2009 Share Posted January 17, 2009 [math]5500 = 25500 \cdot 0.85^x[/math] [math]0.21 = 0.85^x[/math] [math]\log_{0.85} 0.21 = x[/math] [math]\frac{\log 0.21}{\log 0.85} = x[/math] Looks like CrazCo already got that bit. Doesn't matter what the bases are, it's still the right answer... 1 Link to comment Share on other sites More sharing options...
mooeypoo Posted January 17, 2009 Share Posted January 17, 2009 ... when you write "log" with no base, it's as if the base is e, isn't it? In his case the base isn't e, it's a number. It seemed to me to be important.. I might be going ahead of myself here, though, in which case, I apologize. Merged post follows: Consecutive posts mergedOH *SHOOT!* I missed the part where you translated the "base of" the log to log divided by log.. I actually missed my own advice! ha! You're absolutely right, I apologize, the answer's correct as is. Sorry, CrazCo. Link to comment Share on other sites More sharing options...
CrazCo Posted January 19, 2009 Author Share Posted January 19, 2009 no problem thanks Link to comment Share on other sites More sharing options...
Klaynos Posted January 19, 2009 Share Posted January 19, 2009 log with no base implies log10 ln (natural log) is loge Link to comment Share on other sites More sharing options...
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