How to Write Arrays of Arbitrary Formats Without Latex

Question

I need to write an array in the form of the photo below

Answer 1

If you need more than just plain TeX, you can input eplain, so also adding support for color.

\input eplain
\beginpackages
  \usepackage{color}
\endpackages

\definecolor{myred}{RGB}{191,19,18}

$$
\left[\matrix{
  \textcolor{myred}{A_{11}} & A_{12} & A_{13} & \dots & A_{1n} \cr
  A_{21} & \textcolor{myred}{A_{22}} & A_{23} & \dots & A_{2n} \cr
  A_{31} & A_{32} & \textcolor{myred}{A_{33}} & \dots & A_{3n} \cr
  A_{41} & A_{42} & A_{43} & \dots & A_{4n} \cr
  \vdots & \vdots & \vdots & \ddots & \vdots \cr
  A_{n1} & A_{n2} & A_{n3} & \dots & \textcolor{myred}{A_{nn}} \cr
}\right]
$$

\bye

The same with opmac:

\input opmac

\def\myred{\localcolor\setcmykcolor{0 0.9 0.9 0.25}}

$$
\left[\matrix{
  \myred A_{11} & A_{12} & A_{13} & \dots & A_{1n} \cr
  A_{21} & \myred A_{22} & A_{23} & \dots & A_{2n} \cr
  A_{31} & A_{32} & \myred A_{33} & \dots & A_{3n} \cr
  A_{41} & A_{42} & A_{43} & \dots & A_{4n} \cr
  \vdots & \vdots & \vdots & \ddots & \vdots \cr
  A_{n1} & A_{n2} & A_{n3} & \dots & \myred A_{nn} \cr
}\right]
$$

\bye