Maybe this?

\documentclass{article}
\usepackage{tikz}
\newcommand{\cussum}[1]{
    \begin{tikzpicture}[baseline=-.1cm]
    \foreach \x in {1,2,...,#1}
    {
        \ifnum\x<#1
        \node at (\x,0) {$F_{\x}u_{\x}+$};
        \fi
        \ifnum\x=#1
        \node at (\x-.1,0) {$F_{\x}u_{\x}$};
        \fi
    }
    \end{tikzpicture}
}
\begin{document}
    \cussum{9} Minimal Working Examples are nice, aren't they \ldots
\end{document}

Here is the output:

Maybe this?

\documentclass{article}
\usepackage{tikz}
\newcommand{\cussum}[1]{
    \begin{tikzpicture}[baseline=-.1cm]
        \foreach \x in {1,2,...,#1}
        {
            \ifnum\x<#1
                \node at (\x,0) {$F_{\x}u_{\x}+$};
            \fi
            \ifnum\x=#1
                \node at (\x-.1,0) {$F_{\x}u_{\x}$};
            \fi
        }
    \end{tikzpicture}
}
\begin{document}
    \cussum{9} Minimal Working Examples are nice, aren't they \ldots
\end{document}

Here is the output:

Maybe this?

\documentclass{article} \usepackage{tikz} \newcommand{\cussum}1{ \begin{tikzpicture}[baseline=-.1cm] \foreach \x in {1,2,...,#1} { \ifnum\x<#1 \node at (\x,0) {$F_{\x}u_{\x}+$}; \fi \ifnum\x=#1 \node at (\x-.1,0) {$F_{\x}u_{\x}$}; \fi } \end{tikzpicture} } \begin{document} \cussum{9} Minimal Working Examples are nice, aren't they \ldots \end{document}

Here is the output:

Maybe this?

\documentclass{article}
\usepackage{tikz}
\newcommand{\cussum}[1]{
    \begin{tikzpicture}[baseline=-.1cm]
    \foreach \x in {1,2,...,#1}
    {
        \ifnum\x<#1
        \node at (\x,0) {$F_{\x}u_{\x}+$};
        \fi
        \ifnum\x=#1
        \node at (\x-.1,0) {$F_{\x}u_{\x}$};
        \fi
    }
    \end{tikzpicture}
}
\begin{document}
    \cussum{9} Minimal Working Examples are nice, aren't they \ldots
\end{document}

Here is the output: