How to strikeout an flalign block?

Question

I am trying to strikeout an entire flalign block. This post shows how to strikeout an aligned block using the cancel package, but I'm not able to get it to work for the flalign environment. Any ideas how to get this to work without using tikz?

\documentclass{article}
\usepackage{amsmath}
\usepackage[makeroom]{cancel}

\begin{document}
\begin{equation*}
    \cancel{
        \begin{aligned}
        f(x) &= \sin(x) \\
        g(x) &= \cos(x)
    \end{aligned}
}
\end{equation*}
I want to strikeout this:
\begin{flalign*}
    f(x) &= \sin(x) && \\
    g(x) &= \cos(x) &&
\end{flalign*}
\end{document}

Answer 1

You can easily define a left-aligned equation environment and then use the method with the nested aligned.

\documentclass{article}
\usepackage{amsmath}
\usepackage[makeroom]{cancel}

% https://tex.stackexchange.com/a/148854
\makeatletter

\newenvironment{flequation}
  {\@fleqntrue\begin{equation}}
  {\end{equation}\@fleqnfalse}
\newenvironment{flequation*}
  {\@fleqntrue\begin{equation*}}
  {\end{equation*}\@fleqnfalse}

\makeatother

\begin{document}

Some text
\begin{flequation*}
  \cancel{
    \begin{aligned}
      f(x) &= \sin(x) \\
      g(x) &= \cos(x)
    \end{aligned}
  }
\end{flequation*}
More text

\end{document}

Answer 2

One option is to put it in a minipage and strike this out.

\documentclass{article}
\usepackage{amsmath}
\usepackage[makeroom]{cancel}

\begin{document}
\begin{equation*}
    \cancel{
        \begin{aligned}
        f(x) &= \sin(x) \\
        g(x) &= \cos(x)
    \end{aligned}
}
\end{equation*}
I want to strikeout this:\\
\xcancel{\begin{minipage}{2cm}~\\[-2\baselineskip]
\begin{flalign*}
    f(x) &= \sin(x) && \\
    g(x) &= \cos(x) &&
\end{flalign*}
\end{minipage}}
\end{document}

The downside of this is that you have to hard code the width of the minipage. You could cure this with a more complicated macro of by using TikZ.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\newcommand{\tikznode}[3][]{\tikz[remember
picture,baseline=(#2.base)]{\node(#2)[inner sep=0pt,#1]{#3};}}
\begin{document}
\begin{flalign*}
    \tikznode{lt}{\strut}f(x) &= \sin(x) && \\
    g(x) &= \cos(x)\tikznode{br}{\strut} &&
\end{flalign*}
\tikz[overlay,remember picture]{\draw (lt.north) -- (br.south);}
\end{document}

Yet this might be an overkill.