[Tex/LaTex] Underbrace in denominator with () [] {} (howto)

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I would like to put an underbrace correctly under the denominator of a expression but what I get is wrong because of the (). Alternatively I create another line in the text

[~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\underbrace{~~~~~~~~~~~~~~~~~~~~}_{=(1+3\cos^3\theta)}\]

but I don't think that's the right method and the text $(1+3\cos^3\theta)$ yet appear much larger than it should be. The same problem will occur for sqrt for example.

Than I ask, what is the alternative form to comment expressions like these?

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Best Answer

I wouldn't rely on the automated parentheses sizing here anyways, you can set the size up manually:

\documentclass{article}

\usepackage{lipsum}
\begin{document}

\lipsum[1]

\[\biggl(\frac{4\cos^2\theta}{%
\underbrace{4\cos^2\theta + \sin^2\theta}_{{}=(1+3\cos^2\theta)}}
\biggr)
\]

\lipsum[2]

\[\Biggl(\frac{4\cos^2\theta}{%
\underbrace{4\cos^2\theta + \sin^2\theta}_{{}=(1+3\cos^2\theta)}}
\Biggr)
\]

\lipsum[3]

\end{document}

(Use the variant which you like more. For me, it would be the 1st one, for others, it might be the 2nd one.)

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