Grandad, thank you for your reply.

I actually DID mean exactly what I said - I just had the formula wrong - which is what brought me here because I knew something was wrong, I just didn't know what. And I did mention that there might need to be ( ) somewhere in the formula. Thank you for straightening me out though. I also found it somewhere with an m and n, instead of r and s. This was confusing to me because m is often used as the slope.

As for your answer, I almost get it now. Do these two equations have names so that I can look up more on them?

About the weighting and flipping: do you mean we don't have to actually switch the r and s in the formula when P3 moves across the halfway point, right? Because as P3 moves away from P1 and toward P2, the value of the variables representing each side naturally shift their weight to the other.

So for both of these cases:

P1--------------P3-------------------------------P2

|--------r------| |---------------s----------------|

or

P1------------------------------------P3---------P2

|--------r----------------------------| |----s-----|

The same formula applies:

Is this what are saying?

Thank you again,

dmw