\documentclass{article}
\usepackage{pgfplotstable}
\pgfplotsset{compat=1.8}

\pgfplotstablenew[
create on use/x/.style={create col/set list={0,1,...,90}},
create on use/sinx/.style={create col/expr={{sin(\thisrow{x})}}},
create on use/cosx/.style={create col/expr={{cos(\thisrow{x})}}},
create on use/tanx/.style={create col/expr={{tan(\thisrow{x})}}},
columns={x,sinx,cosx,tanx},
]{91}{\mytable}

\begin{document}

\pgfplotstabletypeset[
columns/x/.style={column name=$x$},
columns/sinx/.style={column name=$\sin(x)$},

$\!%
\begin{aligned}
ax^2+bx+c\\
{}+mx+c
	&= ax^2+(b+m)x+c\\
	&= \!%
		 \begin{aligned}[t]
			ax^2+bx+c\\
			{}+mx+c
		 \end{aligned}\\
	&= 0
\end{aligned}
$

$\!%
\begin{aligned}%
ax^2+bx+c\\%
{}+mx+c%
	&=ax^2+(b+m)x+c\\%
	&=\!%
		 \begin{aligned}[t]%
			ax^2+bx+c\\%
			{}+mx+c%
		 \end{aligned}\\%
	&=0%
\end{aligned}%
$

\documentclass[preview,border=12pt]{standalone}
\usepackage{amsmath}
\begin{document}
	$\!
	\begin{aligned}[t]
		-2(2x+3y)(x-2y)\\
		{}-x(x+2)-y^2
			&= \!
				 \begin{aligned}[t]
					-2(2x^2 -4xy +3xy -6y^2)\\
					{}-x^2 -2x -y^2
				 \end{aligned}\\
		 &= \!
				\begin{aligned}[t]
					-2(2x^2 -xy -6y^2)\\
					{}-x^2 -2x -y^2
				\end{aligned}\\
		&= \!
			 \begin{aligned}[t]
					-4x^2 +2xy +12y^2\\
				  {}-x^2 -2x -y^2
			 \end{aligned}\\
		&= -5x^2 -2x +11y^2 +2xy
	\end{aligned}
	$
\end{document}

\documentclass[twocolumn,10pt]{article}
\usepackage[a4paper,margin=5mm]{geometry}
\usepackage{amsmath}
\begin{document}
\begin{enumerate}
	\item
	%$\!%
	\begin{align*}%[t]
		-2(2x+3y)(x-2y)\\
		{}-x(x+2)-y^2
			&= \!
				 \begin{aligned}[t]
					-2(2x^2 -4xy +3xy -6y^2)\\
					{}-x^2 -2x -y^2
				 \end{aligned}\\
		 &= \!
				\begin{aligned}[t]
					-2(2x^2 -xy -6y^2)\\
					{}-x^2 -2x -y^2
				\end{aligned}\\
		&= \!
			 \begin{aligned}[t]
					-4x^2 +2xy +12y^2\\
				  {}-x^2 -2x -y^2
			 \end{aligned}\\

for \textit{k} = \textit{n} + 1, ..., \textit{m} where

\documentclass[10pt]{article}
\usepackage[a6paper,vmargin=15mm,hmargin=5mm]{geometry}
\usepackage{mathtools}

\usepackage{tcolorbox}
\newcounter{exercise}[subsection]
\makeatletter
\@addtoreset{exercise}{section}
\makeatother

\renewcommand{\theexercise}{%
	\ifnum\value{subsection}>0\relax
		\thesubsection
	\else
		\thesection
	\fi
	.\arabic{exercise}%
}

\newenvironment{exercise}
{\par\smallskip\refstepcounter{exercise}
\begin{tcolorbox}[title=Exercise \theexercise]\ignorespaces}
{\end{tcolorbox}\par\smallskip\ignorespacesafterend}

\let\stop\stahp

@DavidCarlisle
I tried to put the tikz \node to variable, but compiler hangs with message: '[Loading MPS to PDF converter (version 2006.09.02).]'

\documentclass{article}
\usepackage{expl3}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\makeatletter
\ExplSyntaxOn
   \tl_new:N \__text
   \protected@edef \__text {\node[draw] (1) {111};}
\ExplSyntaxOff
\makeatletter
\end{tikzpicture}
\end{document}

What's wrong?

@DavidCarlisle @egreg

I tried this:

\begin{tikzpicture}
\ExplSyntaxOn
   \tl_new:N \__text
   \tl_set:Nn \__text {1}
   \tl_put_right:Nn \__text{\node[draw] (1) {\__text};}

   \__text
\ExplSyntaxOff
\end{tikzpicture}

Gives an error:
! Undefined control sequence.
\__text ->1\node
                 [draw](1){\__text };

@DavidCarlisle This works, printing a node with text '1'

\documentclass{article}
\usepackage{expl3}
\usepackage{tikz}
\makeatletter
\ExplSyntaxOn
\tl_new:N \text
\tl_set:Nn \text {1}
\tl_set:Nn \text {\node[draw] (1) {1};}
\ExplSyntaxOff
\makeatother
\begin{document}
\begin{tikzpicture}
\ExplSyntaxOn
\text
\ExplSyntaxOff
\end{tikzpicture}
\end{document}

\documentclass{article}
\usepackage{tikz,xparse}
\begin{document}
\begin{tikzpicture}
\ExplSyntaxOn
   \tl_new:N \l__text_tl
   \tl_set:Nn \l__text_tl {1}
   \tl_put_right:Nx \l__text_tl {\exp_not:n {\node[draw] (1)} {\l__text_tl};}
   \l__text_tl
\ExplSyntaxOff
\end{tikzpicture}
\end{document}

@egreg This one doesn't work, because \node is undefined outside of tikzpicture enviroment:

\documentclass{article}
\usepackage{expl3}
\usepackage{tikz}
\makeatletter
\ExplSyntaxOn
\tl_new:N \text
\tl_set:Nn \text {2}
\protected@edef \text {\node[draw] (1) {\text};}
\ExplSyntaxOff
\makeatother
\begin{document}
\begin{tikzpicture}
\ExplSyntaxOn
\text
\ExplSyntaxOff
\end{tikzpicture}
\end{document}

Is it possible to overcome it?

$ echo $EDITOR