theorem :: EUCLID12:79

for a being Real
for A, B, C being Point of (TOP-REAL 2)
for b, r being Real st A,B,C is_a_triangle & PI < angle (C,B,A) & angle (C,B,A) < 2 * PI & A in circle (a,b,r) & B in circle (a,b,r) & C in circle (a,b,r) holds
( |.(A - B).| / (sin (angle (A,C,B))) = - (2 * r) & |.(B - C).| / (sin (angle (B,A,C))) = - (2 * r) & |.(C - A).| / (sin (angle (C,B,A))) = - (2 * r) )