How to cross-reference an equation without using \label and \ref?

Question

I use

\usepackage{amsmath}
\numberwithin{equation}{chapter}

It means when I make an equation. I just need to do this

\begin{equation}
    equation
\end{equation}

And it's numbered automatically.

So how can I cross-reference the equation numbered (1.2)? I don't \label it, so \ref is not helpful.

Answer 1

\documentclass{book}
\usepackage{amsmath}
\numberwithin{equation}{chapter}

\begin{document}
\chapter{first}
\begin{equation}\label{eq: my very first referenced equation}
    equation
\end{equation}
see \eqref{eq: my very first referenced equation} \dots
\end{document}

You need to run LaTeX at least twice.

Addendum: From comments to your question and changes of question follows, that you actually not interested referencing mechanism ... An alternative is using tags for numbering/tagging of equation and than referencing it manually:

\documentclass{book}
\usepackage{mathtools}% or amsmath ...

\begin{document}
\chapter{first}
\begin{equation}\tag{1.1}
    equation
\end{equation}
see (1.1) \dots
\end{document}

Result is the same as before.

Answer 2

If using \label and \ref (or \eqref) is too inconvenient for you, the leading alternative is to hard-code the cross-reference. E.g., if the equation of interest is numbered "(2.1)", you could write elsewhere

... as was shown in equation (2.1), ...

That said, you'll save yourself a lot of time and effort in the medium to long run if you learn how to use \label and \ref.