[Tex/LaTex] Numerators and denominators in fractions at different height

fractions

I'm trying to write a rather lengthy equation with a fraction multiplied by another fraction that has several \left( and \right) brackets. The height of the symbols above the fraction line is two high in the second fraction. How can I fix this?

\rho_{MN} = \frac{b^2 M}{4\pi} \frac{aR^2+\left[a + 3    \sqrt{z^2+b^2}\right]\left[a+\sqrt{z^2+b^2}\right]^2}{\left\{ R^2+ \left[a+\sqrt{z^2+b^2}\right]^2\right\}^{5/2}\left(z^2+b^2\right)^{3/2}}

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Best Answer

Personally, I would avoid using \left...\right and rather opt for specifying the delimiter sizes via \big-like options. In this specific instance, it suffices to use \bigl...\bigr sizes:

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\documentclass{article}

\usepackage{amsmath}

\begin{document}

\[
  \rho_{MN} = \frac{b^2 M}{4\pi}
              \frac{aR^2 + \left[ a + 3 \sqrt{z^2 + b^2} \right] \left[ a + \sqrt{z^2 + b^2} \right]^2}
                   {\left\{ R^2 + \left[ a + \sqrt{z^2 + b^2} \right]^2 \right\}^{5/2} \left( z^2 + b^2 \right)^{3/2}}
\]

\[
  \rho_{MN} = \frac{b^2 M}{4\pi \vphantom{\big(^{/}}}
              \frac{aR^2 + \bigl( a + 3 \sqrt{z^2 + b^2}\, \bigr) \bigl( a + \sqrt{z^2 + b^2}\, \bigr)^2}
                   {\bigl( R^2 + \bigl(a + \sqrt{z^2 + b^2}\, \bigr)^2 \bigr)^{5/2} \bigl( z^2 + b^2 \bigr)^{3/2}}
\]

\end{document}

The correct placement of the denominator in the left-most fraction is obtained by using a \vphantom of the largest combination of items: a parenthesis together with a superscript - \bigl(^{/}.

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