# Display math should end $$. Missing  inserted. Bad math environment delimiter I have just started writing in Latex and I am getting the same errors constantly. No matter how hard I try to fix these errors, I am breaking some other part. Therefore, I wanted to ask you about my code.  \documentclass[a4paper, 12pt]{article} % preamble \usepackage{a4wide} \usepackage{amsmath} \usepackage{enumerate} \usepackage{nccmath} \begin{document} A body of mass $$m$$ projected straight upward with initial velocity $$v_{0}$$ satisfies the differential equation \begin{equation} m \frac { d v } { d t } = - k v - m g, \hspace{0.5cm} v(0) = $$v_{0}$$ \end{equation} where g = 9.8 m/sec^{2} is the acceleration due to gravity, k > 0 is a constant due to the air resistance, and $$v$$ = v(t\T) is the velocity. The solution is \begin{equation}$$
\begin{center}
$v(t\T)$ = $-\frac{ m g }{ k } + \left( $$v_{0}$$ +$ \frac{ m g }{ k } $\right)$)
$e^{- k t / m }.$
\end{center}
$$\end{equation} \end{document}  My first question is about sizes of fonts. Even though I choose 12 pt, the second equation is being smaller than the first one. Also, I couldn't make the second equation centered as in the first one, therefore, I added \begin{center}...\end{center} to second one. Why it is not being centered as in the first equation? Please help me. Display math should end$$. Missing $inserted. Bad math environment delimiter. How can I eliminate these problems? I would be more than appreciated if you could help me. • In addition to the answer, you should never insert a blank line before a math display, and if the text following the display is part of the same paragraph, don't put a blank line there either. – barbara beeton Oct 12 at 21:59 ## 1 Answer \begin{equation}...\end{equation} already specifies that you're setting math content, so don't use $..$, $$..$$ or $$..$$ inside. \documentclass{article} \usepackage{amsmath} \begin{document} A body of mass$m$projected straight upward with initial velocity$v_0$satisfies the differential equation \begin{equation} m \frac { \mathrm{d} v } { \mathrm{d} t } = - k v - m g, \quad v(0) = v_0 \end{equation} where$g = 9.8 m/s^2$is the acceleration due to gravity,$k > 0$is a constant due to the air resistance, and$v = v(t)\$ is the velocity. The solution is
\begin{equation}
v(t) = -\frac{ m g }{ k } + \left( v_0 + \frac{ m g }{ k } \right) e^{- k t / m }.
\end{equation}

\end{document}

• Thank you so much! I am appreciated to you for your help :) – serife gizem baci Oct 12 at 6:37