php - Annunity calculation before and after tax -


i'm doing annunity calculation through script:

    $calculation=              ( 400000 * 0.02458/( 1- pow( 1+(0.02458) , -84) ) ) ;      return round($calculation,2); 

this return me payment per. month of loan (4.337,76), on period of 84 months.

now, know payment per. month after deduction of 33% tax on interest rate. in previous example (4.337,76) result 4.236,91.

etc. loan before tax:

    month       interest        payment         total     0           815,87          3.561,88        4.377,76     1           807,12          3.570,64        4.377,76     2           798,34          3.579,42        4.377,76 

loan after tax:

    month       interest        payment         total     0           546,63          3.561,88        4.108,52     1           540,77          3.570,64        4.111,41     2           534,89          3.579,42        4.114,30 

as can see, deduction made interest, deducted 33%. ( month 1: (851,86*0.67) = 546,63

i'm there fancy method of doing in annunity calculation, without having calculate full table, fetching every single interest , doing calculation there. cant find it!

looking forward great answers

if i'm understanding correctly want correlate taxes in interest rate right? have say:

r-real = r-nom * (1-t) 

where r-real interest after taxes, r-nom before taxes , t = taxes.

i've pasted quick excel example image.

http://i.stack.imgur.com/wqyxe.png

i hope makes sense.


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