Exponents

What they say on that site is, "This starts to get messy in a hurry. But you can simplify it by noticing that you can keep pulling out factors of (1 + r) from each line. If you do that, the balances collapse to a simple pattern: ..."

That is, even though it's valid, it's a waste of time to pursue that direction; what we've been telling you is the way to go.

If you did continue (and I think someone did so on some thread recently), it would look like this:

P​
P + rP​
(P + rP) + r(P + rP)
[(P + rP) + r(P + rP)] + r[(P + rP) + r(P + rP)]​
([(P + rP) + r(P + rP)] + r[(P + rP) + r(P + rP)]) + r([(P + rP) + r(P + rP)] + r[(P + rP) + r(P + rP)])​

and so on. Do you see how each line is (___) + r(___), where the blank is filled by the line before?

If you were to try to use this for anything, you would want to simplify it -- so why not simplify as you go, which is exactly what we recommend?
 
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