Let' take a simple example:
PRINCIPLE: (cell C3)
INTEREST RATE: (cell D3)

YEAR 1: =SUM(C3+((C3*D3)/100))

OK, everything is fine. Drag it down by itself at you get

=SUM(C4+((C4*D4)/100))

Now you have to go back and change the 4 to a 3.

I have something far more complicated than this. The central problem is the
drag down. I tried holding down the option key but that gives funky results.