How to make number triangles

One option would be to use the gather* environment from amsmath:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{gather*}
  A \\
  B \qquad B \\
  R \qquad R \qquad R \\ 
  A\qquad A \qquad A \qquad A \\ 
  C\qquad C\qquad C\qquad C\qquad C \\
  A\qquad A \qquad A \qquad A \qquad A\qquad A \\
  D\qquad D\qquad D\qquad D\qquad D \\
  A\qquad A \qquad A \qquad A \\
  B \qquad B \qquad B \\
  R \qquad R \\
  A
\end{gather*}

\begin{gather*}
  A \\
  B \quad B \\
  R \quad R \quad R \\ 
  A\quad A \quad A \quad A \\ 
  C\quad C\quad C\quad C\quad C \\
  A\quad A \quad A \quad A \quad A\quad A \\
  D\quad D\quad D\quad D\quad D \\
  A\quad A \quad A \quad A \\
  B \quad B \quad B \\
  R \quad R \\
  A
\end{gather*}

\end{document}

EDIT: a simple TikZ approach:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}

\begin{document}

\begin{tikzpicture}
\matrix[matrix of math nodes,column sep=0pt,inner sep=0pt,row sep=10pt]
{
  &&&&& A \\
  &&&& B && B \\
  &&& R && R && R \\ 
  && A && A && A && A \\ 
  & C && C && C && C && C \\
  A && A && A && A && A && A \\
  & D && D && D && D && D \\
  && A && A && A && A \\ 
  &&& B && B && B \\ 
  &&&& R && R \\
  &&&&& A \\
};
\end{tikzpicture}

\end{document}

EDIT2: a more "pintoresque" design using TikZ:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning,shadows,backgrounds}

\begin{document}
\centering

\begin{tikzpicture}[x=13mm,y=9mm]
  \tikzset{every node/.style={
    minimum height=5mm,
    inner sep=.7mm,
    text width=10mm,
    align=center,
    font=\small\bfseries\sffamily,
    text=olive!50!black,
    draw=olive,
    top color=olive!5,
    bottom color=olive!40,
    rounded corners=2.3mm,
    drop shadow={fill=olive!40!gray,fill opacity=.8}}
}
  \foreach \row/\letterT/\letterB in {0/A/A,1/B/R,2/R/B,3/A/A,4/C/D,5/A} {
    \foreach \col in {0,...,\row} {
        \coordinate (pos) at (-\row/2+\col,-\row);
        \node at (pos) {\letterT};
        \coordinate (posB) at (-\row/2+\col,\row-10); % use: \row-2 times the max. value for \row
        \node at (posB) {\letterB};
    }
  }
\end{tikzpicture}

\end{document}

Put it into a center environment without math

\documentclass{article}
\begin{document}
\begin{center}
A\\
B B\\
R R R\\ 
A A A A\\
C C C C C\\
A A A A A A\\
D D D D D\\
A A A A\\ 
B B B\\ 
R R\\
A
\end{center}

\begin{center}\fontdimen2\font=12pt
A\\
B B\\
R R R\\ 
A A A A\\
C C C C C\\
A A A A A A\\
D D D D D\\
A A A A\\ 
B B B\\ 
R R\\
A
\end{center}

\end{document}