Displaying long equations in multiple lines

Question

I compiled this Tex codes in https://www.latex4technics.com/. The codes are correctly compiled in MathJax but failed in Latex. I am confused about this. How can I have my long equation displayed in multiple lines in Latex? Can I left-align it?

\begin{equation}
  \begin{split}
    \delta_{CNN}=  f_n\left(w_n\ast f_{n-1}\left(\cdots
    \left(w_2\ast f_1\left(w_1\ast\left[
    \begin{matrix}\mu_1\\\mu_2\\\end{matrix}\right]\right. \nonumber \\
    \left.+b_1\right)+b_2\right)\cdots\right)+b_n\right),
  \end{split}
\end{equation}

Answer 1

You don't really need two lines. Here is a simplified code:

\documentclass{article}
\usepackage{amsmath, amssymb}

\begin{document}

\[
 \delta_{CNN}= f_n\left(w_n\ast f_{n-1}\left(\cdots\left(w_2\ast f_1\left(w_1\ast\begin{bmatrix}\mu_1\\ \mu_2\end{bmatrix}%
+b_1\right)+b_2\right)\cdots\right)+b_n\right),
\]

\end{document} 

Answer 2

If you must split the equation across two rows, you should replace all \left auto-sizing directives in row 1 with \Bigl and all \right directives in row 2 with \Bigr. Then, get rid of the remaining \left. and \right. directives.

Incidentally, even if you provided correct number of \right. directives in row 1 and the correct number of \left. directives in row 2, the auto-sizing approach wouldn't generate the desired result because there's no "tall" object in the second row.

Finally, do insert & markers to create alignment points.

\documentclass{article}
\usepackage{amsmath} % for 'split' and 'bmatrix' environments
\begin{document}

\begin{equation}
  \begin{split}
    \delta_{\mathrm{CNN}}
    &=  f_n\Bigl(w_n\ast f_{n-1}\Bigl(\cdots\Bigl(w_2\ast f_1\Bigl(w_1\ast
        \begin{bmatrix} \mu_1 \\ \mu_2 \end{bmatrix}    \\[1ex]
    &\quad +b_1\Bigr)+b_2\Bigr)\cdots\Bigr)+b_n\Bigr)\,.
  \end{split}
\end{equation}
\end {document}