4. Given a triangle $A B C$, where side $B C$ is twice the length of side $A B$, and a bisector $B M$ is drawn. a) Find the ratio $R_{1} / R_{2}$, where $R_{1}, R_{2}$ are the radii of the circumcircles of triangles $A B M$ and $C B M$ respectively. b) Prove that $3 / 4 < r_{1} / r_{2} < 1$, where $r_{1}, r_{2}$ are the radii of the incircles of triangles $A B M$ and $C B M$ respectively.