Parenthesis Size in a multiline equation

Question

How can I use

\left(.... \right)

which automatically adjust size of parenthesis with it's inside content, in a multiline equation in

\begin{align} and \begin{equation}

environment.

Answer 1

You can, as documented in mathtools, thanks to a code by Sébastien Gouezel (§3.6.1 of the doc):

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}
\usepackage{lmodern}

\newcommand\MTkillspecial[1]{% helper macro
\bgroup
\catcode`\&=9
\let\\\relax%
\scantokens{#1}%
\egroup
}
\DeclarePairedDelimiter\brparen
\lparen\rparen
\reDeclarePairedDelimiterInnerWrapper\brparen{star}{
\mathopen{#1\vphantom{\MTkillspecial{#2}}\kern-\nulldelimiterspace\right.}
#2
\mathclose{\left.\kern-\nulldelimiterspace\vphantom{\MTkillspecial{#2}}#3}}

\begin{document}
\begin{equation}
A =\! \begin{aligned}[t]
  \brparen[\Big]{& a + B + c + d + e{} \\
         & + f +g + h}\
\end{aligned}
\end{equation}

\begin{equation}
A =\! \begin{aligned}[t]
  \brparen*{& \frac{K^2}{L^2} + B + + C + D + E\\
         & + F + G + H}
\end{aligned}
\end{equation}
\end{document}