I must be misunderstanding what you mean by "interval bisection". The point bisecting the interval from [itex](x_0,y_0,z_0)[/itex] to [itex](x_1, y_1, z_1)[/itex], in 3 dimensions, is [itex]((x_0+ x_1)/2, (y_0+ y_1)/2, (z_0+ z_1)/2)[/itex], "averaging" the coordinates. That can be derived by using 'similar triangles' but you surely don't have to rederive it for each application.