I'm not sure if this should be here or in geometry, but...

I have two lines A and B, and I know their length. Thus I can get A + B = L0. I now have L1. How do I find the length of A and B such that A/B remains the same?

Another way to look at it is that all the following must be true, with A, B, and F all known and I'm looking for C and D.

A + B = N

A / B = R

C + D = F

C / D = R