How to put an qedsymbol at the end of an array environment?

Question

I want to put an qedsymbol at the end of a non-proof environment such as

\begin{theorem}
This is a funny equation
\[
\begin{array}{rcl}
x+y&=&z,\\
a+b&=&c.
\end{array}
\]
\end{theorem}

and I want to put the symbol at the right side of the line of

a+b=c

but not the right side of the whole list of equations.

And also in a proof environment I want to make qedsymbol at the right side of the line of a+b=c but not the right side of the whole list of equations.

\begin{proof}
So we have
\[
\begin{array}{rcl}
x_1+y^2&=&z^3,\\
a^2+b_2&=&c^6.
\end{array}
\]
\end{proof}

QUESTION: Who has a good way to do these two things together?

All the codes of mine are as follows.

\documentclass[11pt,b5paper]{ctexbook}
\usepackage{amsmath,amsthm,amssymb,amsfonts,mathrsfs}
%%%change qedsymbol%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\renewcommand{\qedsymbol}{\ensuremath\boxtimes}
%%%set theorems%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\theoremstyle{theorem}
\newtheorem{theorem}{定理}[section]
\newtheorem{lemma}[theorem]{引理}
\newtheorem{corollary}[theorem]{推论}
\newtheorem{proposition}[theorem]{命题}
\theoremstyle{definition}
\newtheorem{definition}[theorem]{定义}
\newtheorem{remark}[theorem]{评注}
\newtheorem{example}[theorem]{示例}
\newtheorem{exercise}[theorem]{习题}
\newtheorem{thesis}[theorem]{论题}
\newtheorem{conjecture}[theorem]{猜想}
\newtheorem{convention}[theorem]{约定}
\newtheorem{problem}[theorem]{问题}
\newtheorem{axiom}[theorem]{公理}    
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

\begin{theorem}
This is a funny equation
\[
\begin{array}{rcl}
x+y&=&z,\\
a+b&=&c.\qedhere
\end{array}
\]
\end{theorem}

\begin{proof}
So we have
\[
\begin{array}{rcl}
x_1+y^2&=&z^3,\\
a^2+b_2&=&c^6.\qedhere
\end{array}
\]
\end{proof}


\begin{proof}
So we have
\begin{align*}
x+y&=z,\\
a+b&=c.\qedhere
\end{align*}
\end{proof}

\end{document}

The compiled PDF sample is also as follows.

NOTE: the new command \myqed I defined can only work in the equation environment (with only 1 line of equations), but can't work in the align* environment. So I still haven't solve the problem.

Answer 1

I fixed your code:

\documentclass[11pt,b5paper]{ctexbook}
\usepackage{amsmath,amsthm,amssymb,amsfonts,mathrsfs}
%%%change qedsymbol%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\renewcommand{\qedsymbol}{\ensuremath\boxtimes}
%%%set theorems%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\theoremstyle{theorem}
\newtheorem{theorem}{定理}[section]
\newtheorem{lemma}[theorem]{引理}
\newtheorem{corollary}[theorem]{推论}
\newtheorem{proposition}[theorem]{命题}
\theoremstyle{definition}
\newtheorem{definition}[theorem]{定义}
\newtheorem{remark}[theorem]{评注}
\newtheorem{example}[theorem]{示例}
\newtheorem{exercise}[theorem]{习题}
\newtheorem{thesis}[theorem]{论题}
\newtheorem{conjecture}[theorem]{猜想}
\newtheorem{convention}[theorem]{约定}
\newtheorem{problem}[theorem]{问题}
\newtheorem{axiom}[theorem]{公理}    
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

\begin{theorem}
This is a funny equation
\begin{align*}
x+y&=z,\\
a+b&=c.\qedhere
\end{align*}
\end{theorem}

\begin{proof}
So we have
\begin{align*}
x_1+y^2&=z^3,\\
a^2+b_2&=c^6.\qedhere
\end{align*}
\end{proof}


\begin{proof}
So we have
\begin{align*}
x+y&=z,\\
a+b&=c.\qedhere
\end{align*}
\end{proof}

\end{document}

If array environment is the last object in the theorem or proof environment, then you should do this:

\begin{theorem}
This is a funny equation
\[
\begin{array}[b]{...}
....
\end{array}\qedhere
\]
\end{theorem}

Answer 2

\documentclass{article}
\usepackage{amsmath,amsthm}
\newtheorem{thm}{Theorem}
\newenvironment{theorem}[1][]{\begin{thm}[#1]\pushQED{\qed}}{\popQED\end{thm}}
\begin{document}
\begin{theorem}
This is a funny equation
\begin{align*}
x+y&=z,\\
a+b&=c.\qedhere
\end{align*}
\end{theorem}

\begin{proof}
So we have
\begin{align*}
x+y&=z,\\
a+b&=c.\qedhere
\end{align*}
\end{proof}
\end{document}