Missing $ inserted \end{frame}

Question

I am writing a beamer. I got lots of Missing $ inserted \end{frame}. Where I havent missed any $. this is the part of the code I have used where similar error is shown

\begin{frame}
\begin{normalsize}
Sectors and angle of each sectors are shown in Table 1:

\begin{table}[hb]
\caption{Sectors and Sector Angle}
\label{Sectors and Sector Angle}
\vspace{0.35cm}
\centering
\begin{tabular}{|c|c|}
\hline \textbf{SECTOR} & \textbf{DEGREE} \\ 
\hline Sector 1 & -30\leq\theta_s\leq30 \\ 
\hline Sector 2 & 30\leq\theta_s\leq 90 \\ 
\hline Sector 3 & 90\leq\theta_s\leq 150 \\ 
\hline Sector 4 & 150\leq\theta_s\leq-150 \\ 
\hline Sector 5  & -150\leq\theta_s\leq-90 \\ 
\hline Sector 6 & -90\leq\theta_s\leq-30\\ 
\hline 
\end{tabular}
\end{table}
\end{normalsize}
\end{frame}

Answer 1

The right column in your table consists mainly of math formulas, so you need to add $ to the column specification. Instead of inserting $..$ at each cell, you can save typing by modifying the column specification as \begin{tabular}{|c|>{$}c<{$}|}.

A better visual alignment is to make the term theta_s at the center of the column, this is achieved via \begin{tabular}{|c|>{$}r<{$} @{${}\leq\theta_s\leq{}$} >{$}l<{$}|}. Now we have split the right column into two r and l and the expression {}\leq\theta_s\leq{} at the center. The two {} are there just to correct spacing around the binary operator \leq.

\documentclass[12pt,a4paper]{beamer}
\usepackage{lmodern,array}
\begin{document}

\begin{frame}
\begin{normalsize}
Sectors and angle of each sectors are shown in Table 1:

\begin{table}[hb]
\caption{Sectors and Sector Angle}
\label{Sectors and Sector Angle}
\vspace{0.35cm}
\centering
\begin{tabular}{|c|>{$}r<{$} @{${}\leq\theta_s\leq{}$} >{$}l<{$}|}
\hline \textbf{SECTOR} & \multicolumn{2}{c|}{\textbf{DEGREE}} \\ 
\hline Sector 1 &  -30 &   30 \\ 
\hline Sector 2 &   30 &   90 \\ 
\hline Sector 3 &   90 &  150 \\ 
\hline Sector 4 &  150 & -150 \\ 
\hline Sector 5 & -150 &  -90 \\ 
\hline Sector 6 &  -90 &  -30 \\ 
\hline 
\end{tabular}
\end{table}
\end{normalsize}
\end{frame}

\end{document}

Answer 2

AboAmmar has explained how to fix the problem; this is a complementary answer, which explains the error message.

TeX generates the same error message, Missing $ inserted, for two very different circumstances:

• You left out the closing $ on a math expression. This error gets reported at the point where you leave the "group" construct or paragraph that contains the opening dollar sign for the math expression.

• You left out the opening $. This error gets reported at the point where you use a TeX primitive that is only allowed in math mode.

In both cases, the place where the $ should have been is almost always some distance before the place where TeX reports the error.

The error message you got looks like it's complaining about a missing closing $, because it's associated with \end{frame}, but the mistake you actually made is to write

-30\leq\theta_s\leq30

instead of

$-30\leq\theta_s\leq30$

(and similarly for the other rows) because \leq, \theta, and the subscript character _ can only be used in math mode.

I don't know why you only got a complaint at the point of \end{frame}; probably Beamer and/or the table machinery are doing something which causes the error not to be reported promptly.