From: John Butler <johnnybutler7@...>
Date: 2007-03-30T18:21:50+09:00
Subject: Re: Ruby math.pow equivalent

Drew Olson wrote:
> John Butler wrote:
>> Hi,
>> 
>> I am trying to calculate a monthly payment for a loan in Ruby.  Does
>> anyone know what the equivalent for math.pow in Java is for Ruby?
>> 
>> @monthlypayment = @principal*@rate/(1-Math.pow(1/(1+@rate),@payments))
>> 
>> Ive searched everywhere and cant seem to find anything.
>> 
>> thanks
>> 
>> Johnny B
> 
> I think this is what you're looking for:
> 
> irb(main):001:0> 2 ** 1
> => 2
> irb(main):002:0> 2 ** 2
> => 4
> irb(main):003:0> 2 ** 3
> => 8
> irb(main):004:0> 2 ** 4
> => 16



Yes this is what i am looking for. How do i then use it to calculate 
amonthly payment, i have tried quite a few things but cant get the 
syntax right: @monthlypayment = 
@principal*@rate/(1-**(1/(1+@rate),@payments))


So for the example below:

Suppose you finance your car with a loan of $12000 at a yearly interest 
rate of 11% for four years, and make equal payments monthly. How much 
will your payments have to be? Here the parameters are principal P = 
$12000, interest rate i = 0.11, number of years n = 4, and number of 
periods per year q = 12. Then the monthly car payment M is given by

    M = Pi/[q(1-[1+(i/q)]-nq)],
      = ($12000)(0.11)/[(12)(1-[1+(0.11/12)]-(4)(12))],
      = $110/(1-1.009166666...-48),
      = $310.15.


Anyone done this before?


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