theorem :: EUCLID10:35

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