The Cosine Rule for Hyperbolic Triangles For any htriangle ABC, sinh(b)sinh(c)cos(A) = cosh(b)cosh(c)  cosh(a), with similar formulae for cos(B) and cos(C).
Proof 

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 se^{iA},
From (3), This immediately gives the required result.

by the Basic Strategy 