theorem Thm20: :: EUCLID10:31

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