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by: Michael2009Posted on 2009-08-21 at 11:57:22ID: 25154729
Let's say payments are figured in before interest is calculated.
2)^M-(1+R/ 12))/(R/12 )
So for example, on the first month, we take the payment off the principal, then we calculate interest, and the result is the new balance. Let's call the monthly payment P.
In this case, the recursive formula is (bear with me, as subscripts are hard to write here):
B(n+1) = (Bn-P)*(1+R/12)
In other words, this month's balance is last month's minus the payment, times interest.
If this is the case, the formula for the balance after M months is:
(S-P)*(1+R/12)^M - P* [ (1+R/12)^M - (1+R/12) ] / (R/12)
I'm not using the variable N which you gave, because that needs to be used to figure the payment P. The way you do this depends on the bank and/or the person paying.
In Excel terms, simply replace all the letter variables in the equation with references to the cells that contain the values you need. Also, the square brackets need to be changed to parentheses.
So it would be something like:
(S-P)*(1+R/12)^M-P*((1+R/1