I have a problem that is more of a math problem than excel.
I want to show an annual loan interest calculation that is based on a
monthly amortization schedule.
The problem is that each month we repay some of the principal and then next
months interest is calculated on a different principal amount.
Illustrated I am imagining that this is how this would look assuming we
start with a balance of $1,000 and an monthly interest rate of r.

1,000*r+(1,000-((1,000*r))*r+(1,000-(1,000-((1,000*r))*r)...and so on

This seems like some sort of math series and I am wondering if this can be
simplified in a formula.

If anyone can help I would really appreciate it.
Thanks.

Neda