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April 29th, 2013, 09:11 AM  #1 
Newbie Joined: Apr 2013 Posts: 2 Thanks: 0  Help with APR calculation
Hello everyone, I am trying to determine the interest rate based on this monthly payment formula: M = P(1+r)n r / [(1+r)n1] Can anyone solve for r? Thanks in advance. 
April 29th, 2013, 09:33 AM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 521 Math Focus: Calculus/ODEs  Re: Help with APR calculation
Please post a topic only once, as doing so multiple times runs the risk of duplication of effort on the part of our contributors, whose time is valuable. Your formula doesn't look correct...shouldn't you have some exponents? 
April 29th, 2013, 09:42 AM  #3 
Newbie Joined: Apr 2013 Posts: 2 Thanks: 0  Re: Help with APR calculation
Sorry, didn't have the formula in the right format. M = P(1+r)^n*r / [(1+r)^n1] I got the formula from this website: http://math.fau.edu/qian/course/MGF1...%20formula.htm 
April 29th, 2013, 07:04 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 20,978 Thanks: 2229 
Which would you prefer: (a) a neat formula which is approximate, may not be accurate enough for you, and is different for each value of n or (b) a method of approximating r that is tedious and repetitive, but gives you whatever accuracy you require if repeated often enough?

April 29th, 2013, 08:17 PM  #5 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 14,597 Thanks: 1038  Re: Help with APR calculation
Formula is less "wieldy" this way: M = Pr / [1  (1 + r)^(n)] 

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