Question 51742
assume   FG + GJ = FH + HJ  true

 FG + GJ = FH + HJ 
FG + GH = FH(cuz G is between F&H)
GH + HJ = GJ ("  H is between G&J)                                         


GH+HJ+FG= FG+GH+HJ  (FG + GH = FH ;GH + HJ = GJ ;proved)
Q.E.D
So its true