\usepackage[sfdefault]{roboto}  %% Option 'sfdefault' only if the base font of the document is to be sans serif
\usepackage[T1]{fontenc}
\mdseries
\documentclass{article}
\begin{document}
\begin{math}
  \psi(x_b, t_b) = \int_{-\infty}^{+\infty} K(b, a)\psi(x_a, t_a)dx_a
  \end{math}
%% K(b, a) = (\frac{m}{ih(t_b - t_a)})^\frac{1}{2} exp \frac{im(x_b - x_a)^2}{2\hbar(t_b - t_a)}
\end{document}