usepackage[all]{nowidow}

begin{dmath}
.
.
end{dmath}

documentclass[12pt,a4paper]{article}
usepackage{blindtext}
usepackage{breqn}
usepackage[all]{nowidow}

begin{document}

blindtext
%
begin{dmath}
(dot{x}_i, dot{y}_i) cdot mathbf{n}_i approx mathrm{i}omega left( htheta_2sin{(alpha_i + beta_i)} + r_i(theta_1 + theta_2)sin{(2alpha_i + beta_i)}
- frac{y_L + y_R}{2} cos{(alpha_i + beta_i)} right)
end{dmath}
%
blindtext

end{document}