Hi Rich, Thanks for valuable insight,
source was my previous assignment ( where i took classes,while back) unfortunately i lost explanation sheet, i googled it and didn't found ,newhere


as far as q concerned

Whether S is in between R and T?
(1) | R - T | > | R - S |
(2) | R - T | > | T - S |

from 1. r^2 +t^2 -rt > r^2+s^2-rs
t+s > r
from here i spred these variables on number line
0-----r---t------------s NO
0-----r----s-----------t
both satisfies "t+s > r" both give different result , hence NO for stem 1
same for stem 2

I dont know whether it is clear concise sure short method, it took couple of minutes, thanks once again