documentclass[11pt,a4paper]{report}
usepackage[utf8]{inputenc}
usepackage[english]{babel}
usepackage[T1]{fontenc}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
usepackage[T1]{fontenc}
usepackage[black]{merriweather}
usepackage{lipsum}
begin{document}

chapter{Introduction}
lipsum[2] $x=4$.

begin{displaymath}
Delta G^{circ}= -R,Tln dfrac{[S_1][S_2]}{[S_1S_2]}.
end{displaymath}

end{document}