Problem with text position in math underbrace

Question

I'm trying to write under a system of equation that it is impossible with the following code:

\[\underbrace{\begin{cases}
a = -1\\
b=2\\
(-1-4)-i(2+8)=-8i\implies -5 -10i = -8i
\end{cases}_{\text{Sistema impossibile}}}\bigcup\begin{cases}
a=0\\
b=2\\
-i(2^3)=-8i
\end{cases}\]

The problem is that the text gets positioned badly as you can see in the pic:

How can I get the text to actually be under the brace and not there? Thanks in advance for the answers.

Answer 1

An \underbrace positions its label the way the \sum operator gets its lower limit: entered as a subscript. In the code sample the subscript label comes right after \end{cases} instead of after the whole \underbrace{...} due to a misplaced close-brace character (so the intended label was an ordinary subscript for the cases environment). Moving the brace to come before the subscript fixes the trouble, as in this MWE.

\documentclass[12pt]{article}
\usepackage{amsmath,amssymb}

\begin{document}
\[\underbrace{\begin{cases}
a = -1\\
b=2\\
(-1-4)-i(2+8)=-8i\implies -5 -10i = -8i
\end{cases}}_{\text{Sistema impossibile}}\bigcup
\begin{cases}
a=0\\
b=2\\
-i(2^3)=-8i
\end{cases}\]
\end{document}

---

Addendum with the \lor command as by comment of the user @egreg.

\documentclass[12pt]{article}
\usepackage{amsmath,amssymb}

\begin{document}
\[\underbrace{\begin{cases}
a = -1\\
b=2\\
(-1-4)-i(2+8)=-8i\implies -5 -10i = -8i
\end{cases}}_{\text{Sistema impossibile}}\lor
\begin{cases}
a=0\\
b=2\\
-i(2^3)=-8i
\end{cases}\]
\end{document}