AS3 doesn't like my formula

I am trying to code a loan calculator into Flash, however AS3 doesn’t like my formula and won’t calculate it, can someone help? Here is what I have:

loan=15000
apr=13
term=48

var loan_amount = (apr + (apr / (Math.pow(trm, (1 + apr)) -1 ))) * loan;

When I print out the loan_amount I get a 0.

If I do: var loan_amount = (Math.pow(trm, (1 + apr)) -1); it will give me the result 1.0331079714165e55

As soon as I add the next part of the equation (apr / (Math.pow(trm, (1 + apr)) -1 )); it will give me the actual apr value as the result instead of the apr divided by the 1.0331079714165e55.

Likewise, if I try to add a fixed value instead of the apr variable, it will return that fixed value instead of doing the math.

Can anyone help me with getting this to work?

Thanks!
Anthony