theorem Th12: :: JORDAN6:12

for P being Subset of (TOP-REAL 2)
for p1, p2 being Point of (TOP-REAL 2) st P is_an_arc_of p1,p2 holds
ex q being Point of (TOP-REAL 2) st
( q in P & q `1 = ((p1 `1) + (p2 `1)) / 2 )