function x=cubica(v,ep)
if v < 0
    signo = -1;
else
    signo = 1;
end
    v = abs(v);
 k=1;
 while true
    x=(1./3).*((v./k.^2)+2*k);
    if abs((v.^(1./3)-x)/v.^(1./3))*100<=ep;
        break
    else
        k=k+0.01;
    end
 end
    x = x * signo;
end