%%
%% The LaTeX Companion, 3ed
%%
%% Example 12-3-27 on page II-249 in "newpxmath --- Using the PX fonts for math".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{207.0pt}
%StartShownPreambleCommands
\usepackage{amsmath,newpxtext,newpxmath}
%StopShownPreambleCommands

\begin{document}
A better result:
\[ t[u_1, \dots, u_n] = \sum_{k=1}^n
   \binom{n-1}{k-1} (1-t)^{n-k}t^{k-1}u_k \]
\end{document}