The Cosine Rule for Hyperbolic Triangles
For any h-triangle ABC,
sinh(b)sinh(c)cos(A) = cosh(b)cosh(c) - cosh(a),
with similar formulae for cos(B) and cos(C).
assume that A is O, and that B is on the positive real axis.
As A = O, AB and AC are line segments, making the angle A.
Thus the complex coordinates of A, B and C are 0, r and seiA,
This immediately gives the required result.
by the Basic Strategy