How to control the alignment of the brace in a dcases environment

Question

The following code

\begin{equation}
    \begin{dcases}
        u = \frac{\sqrt{2}}{2}(i+j)\\
        v = k
    \end{dcases}
\end{equation}  

results in the following display

that I find particularly disgraceful. Is there a way to lower the brace to make it look more like this

?

Answer 1

You can "smash" the fraction from the top using \smash[t]. Like this:

\documentclass{article}
\usepackage{mathtools}
\begin{document}

\begin{equation}
    \begin{dcases}
        u = \smash[t]{\frac{\sqrt{2}}{2}(i+j)}\\
        v = k
    \end{dcases}
\end{equation}  

\end{document}

The command \smash[b] would be useful for the last line.

---

Workaround: since I don't suppose you need it regularly, a manual tweak suggested by egreg is possible. Here, we increase the vertical space above the equation, and by grouping we ensure that the effect will be local.

\begingroup\addtolength{\abovedisplayskip}{4pt}\begin{equation}
    \begin{dcases}
        u = \smash[t]{\frac{\sqrt{2}}{2}}(i+j)\\
        v = k
    \end{dcases}
\end{equation}\endgroup