Curved line in formulas

Question

I want to write a column of formulas, but with a curved line on the left side, like on that picture. How to do it?

Answer 1

Here is something you can try using tikz.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing}

\begin{document}

\begin{tikzpicture}[thick] 
\draw [decorate, 
      decoration={snake, segment length=10mm, amplitude=2mm}
      ] (0,.5) -- (0,4.5);
  \node [anchor=west] at (1, 4) {$(a+b)^2 = a^2 + 2ab + b^2$};
  \node [anchor=west] at (1, 3) {$(a-b)^2 = a^2 - 2ab + b^2$};
  \node [anchor=west] at (1, 2) {$(a+b)(a-b) = a^2 - b^2$};
  \node [anchor=west] at (1, 1) {$(a + b + c)^2 = a^2 + b^2 + c^2 + 2(ab + bc + ca)$};
\end{tikzpicture}

\end{document}

Output:

Note: You could adjust segment length=10mm and amplitude=2mm to adjust the curves as required.

Answer 2

We can use the MnSymbol and its \lwavy, \rwavy, \lWavy, \rWavy extensible delimiters, with empheq:

\documentclass{article}
\usepackage[utf8]{inputenc}
 \usepackage[T1]{fontenc}
\usepackage{MnSymbol}
 \usepackage{empheq}
\DeclareLeftDelimiter{\lwavy}
\DeclareRightDelimiter{\rwavy}

\begin{document}

\begin{empheq}[left =\boldsymbol\empheqlwavy\quad]{align*}
    (a+b)^2 & = a^2 + 2ab + b^2; \\
    (a-b)^2 & = a^2 - 2ab + b^2; \\
    (a+b)(a-b) & = a^2 - b^2; \\
    (a + b + c)^2 & = a^2 + b^2 + c^2 + 2(ab + bc + ca)
\end{empheq}

\end{document}