How to make formulae take equal vertical space in the align environment?

Question

Consider the following code:

\begin{align}
f_1(x) &= \frac{15x}{3} \\
f_2(x) &= 3x + 5 \\
f_3(x) &= 4x + 13
\end{align}

This produces three lines where the first line takes up more vertical space than the others. Is there any way to make all the lines take the same vertical space (meaning effectively all lines take as much vertical space as the largest one)?

Answer 1

I don't know of an automatic way to do this, but in a particular case, you can use \vphantom to effect this.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
f_1(x) &= \frac{15x}{3} \\
f_2(x) &= \vphantom{\frac11}3x + 5 \\
f_3(x) &= \vphantom{\frac11}4x + 13
\end{align}
\end{document}

Answer 2

if you do not want it globally changed put the complete formula into {...}

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

\savebox\strutbox{$\vphantom{\dfrac11}$}
\begin{align}
  f_1(x) &= \frac{15x}{3} \\
  f_2(x) &= 3x + 5 \\
  f_3(x) &= 4x + 13
\end{align}

\end{document}

Answer 3

I can offer a variant of TH's answer that produces a bit less vertical space, but it is adjusted to your particular situation: Only the second line needs a bit of additional depth. The advantage of this more special adjustment is that it doesn't produce additional white space below the display.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
Here's some text just to fill the line sufficiently
\begin{align}
f_1(x) &= \frac{15x}{3} \\
f_2(x) &= 3x + 5 \vphantom{\smash[t]{\frac11}} \\
f_3(x) &= 4x + 13
\end{align}
Here's some text just to fill the line sufficiently
\end{document}