function [xg_tang_1,yg_tang_1] = f_tangeant(xa,ya,rt,rg)

delta = 4*( (xa^2)*(ya^2)-(((xa^2)-(rt^2))*((ya^2)-(rt^2))) );

a_1_1=( (2*xa*ya)+sqrt(delta) ) / ( 2*((xa^2)-(rt^2)) );
a_1_2=( (2*xa*ya)-sqrt(delta) ) / ( 2*((xa^2)-(rt^2)) );

b_1_1=rt*sqrt(1+a_1_1^2);
b_1_2=rt*sqrt(1+a_1_2^2);

if (xa>0)
   [flag,xg_tang_1,yg_tang_1]=f_interbis(a_1_2,b_1_2,xa,ya,rg);
else
   [flag,xg_tang_1,yg_tang_1]=f_interbis(a_1_1,b_1_1,xa,ya,rg);
end