\nonstopmode
\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{cancel}
\pagestyle{empty}
\begin{document}
$$
{\mathcal  {D}}\equiv {\biggl (}{\frac  {b_{1}}{3}}{\biggr )}^{2}+{\biggl (}{\frac  {b_{0}}{2}}{\biggr )}^{2}={\biggl (}{\frac  {4a_{0}}{3}}{\biggr )}^{2}+{\biggl (}{\frac  {a_{1}^{2}}{2}}{\biggr )}^{2}={\biggl (}{\frac  {a_{1}^{2}}{2}}{\biggr )}^{2}{\biggl [}1+{\biggl (}{\frac  {8a_{0}}{3a_{1}^{2}}}{\biggr )}^{2}{\biggr ]},
$$
\end{document}
