How can I input endmark?

Question

This is my code

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage[thmmarks,standard,thref]{ntheorem}
\usepackage{graphicx}
\usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}
\theoremseparator{.}
\theoremsymbol{\ensuremath{\square}}
\newtheorem{sol}{Soluttion}

\begin{document}
\begin{sol}
This is an exercise
\end{sol}


\begin{sol}
This is an exercise
\begin{equation}
x^2 - 2(m+1)x - m^2 - 1 = 0.
\end{equation}
\end{sol}
\end{document}

I input endmark, but after \begin{equation} \end{equation} I can not receive endmark.

Answer 1

what you want is already provided by the proof environment of amsthm.

try the following (simplified from your example):

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb} % loads amsfonts automatically

\begin{document}
\begin{proof}[Solution]
This is an exercise
\end{proof}

\begin{proof}[Solution]
This is an exercise
\begin{equation}
x^2 - 2(m+1)x - m^2 - 1 = 0.
\qedhere
\end{equation}
\end{proof}
\end{document}

the proof heading can be changed to "Solution" by adding the code

\renewcommand{\proofname}{Solution}

in the preamble.

only the proof environment is set up to recognize \qedhere. adding this facility to theorem-class environments is on the ams list of requests for consideration, but isn't likely to be looked at for a while.