theorem Th28: :: EUCLID13:35

for A, B, C being Point of (TOP-REAL 2) st A,B,C is_a_triangle & angle (B,A,C) < PI holds
( - PI < (angle (B,A,C)) - (angle (C,B,A)) & (angle (B,A,C)) - (angle (C,B,A)) < PI )