Fraction at the same height

Question

\begin{equation} \label{eq:pressure3}
\cfrac{P_2}{P_0} = \cfrac{C_2}{C_2 + S^*_2}\cdot\cfrac{C_1}{C_1+S^*_1+\cfrac{C_2 S^*_2}{C_2+S^*_2}}
\end{equation}

How can I bring the first denominator at the same height as the second denominator?

Answer 1

(see FOLLOW UP below for a more general approach)

Here a \vphantom of the tall term, added to the first term, can help.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation} \label{eq:pressure3}
\cfrac{P_2}{P_0} = \cfrac{C_2}{C_2 + S^*_2\vphantom{\cfrac{S^*_2}{S^*_2}}}
  \cdot\cfrac{C_1}{C_1+S^*_1+\cfrac{C_2 S^*_2}{C_2+S^*_2}}
\end{equation}
\end{document}

FOLLOW UP

In response to David's comment, rather than over-exercising the \vphantom approach above, I introduce \sfrac which uses the values of \topgap and \botgap as baselineskips to the numerator and denominator, regardless of the height of those quantities. It thus avoids the use of \vphantoms.

It would be intended to use on the first level of all terms of the fraction.

\documentclass{article}
\usepackage{amsmath,stackengine}
\def\topgap{7pt}
\def\botgap{\topgap}
\newcommand\sfrac[2]{\def\stacktype{L}\ensurestackMath{%
  \stackunder[\botgap]{\stackon[\topgap]{\dfrac{\phantom{#1}}{\phantom{#2}}}{#1}}{#2}%
}}
\begin{document}
\begin{equation}
\def\botgap{15pt}
\sfrac{P_2}{P_0} = \sfrac{C_2}{C_2 + S^*_2}
  \cdot\sfrac{C_1}{C_1+S^*_1+\cfrac{C_2 S^*_2}{C_2+S^*_2}}
\end{equation}
\end{document}