function rou_k=solve_rou_k(Xk,f,df,pk,Bk)

pt2=Xk(1,1);
pt3=Xk(2,1);
pr2=Xk(3,1);
pr3=Xk(4,1);


f_old=eval(f);

gk=eval(df);

temp=Xk+pk;
pt2=temp(1,1);
pt3=temp(2,1);
pr2=temp(3,1);
pr3=temp(4,1);



f_new=eval(f);

rou_k=(f_old-f_new)/(-0.5*pk.'*Bk*pk-gk.'*pk);



