# Aligning equations: equation+system of equations on the same line

I have a line on which there are an equation and a system of equations. How can I align the equations of the system (vertical alignment of = symbols)?

\documentclass{article}

\usepackage{amsmath}

\newenvironment{sistema}%
{\left\lbrace\begin{array}{@{}l@{}}}%
{\end{array}\right.}

\begin{document}

\begin{equation*}
TTR(t) = TTR_{PLATEAU} \left( 1 - e^{- k \, t} \right) \hspace{0.5cm} con \hspace{0.2cm} \begin{sistema}
TTR_{PLATEAU} = \frac{ir}{R_a} \\
k = \frac{R_a}{Q}
\end{sistema}
\end{equation*}

\end{document}


Thank you so much for your help.

• How is the sistema environment defined?
– Mico
Dec 3 '17 at 8:18
• Sorry @Mico, now I edit my question. Dec 3 '17 at 8:19

I can think of two solutions -- see below. Speaking for myself, I'd say the first solution, which doesn't perform alignment on the = symbols inside the "sistema", looks better. However, there's no arguing about tastes, right? Note that both solutions typeset the material of the systems in display-stye math mode.

Either way, do take care to properly typeset variable names, such as TTR and PLATEAU. The following example provides some suggestions in this regard.

\documentclass{article}
\usepackage{amsmath,array}
\newcolumntype{L}{>{\displaystyle}l} % left-aligned, automatic display-math style
\newcolumntype{R}{>{\displaystyle}r}
\newcolumntype{C}{>{{}}c<{{}}}

\newcommand\TTR{\textit{TTR}}
\newcommand{\PLATEAU}{\textsc{plateau}}

\newenvironment{sistema}%
{\setlength\arraycolsep{0pt}
\left\lbrace \begin{array}{L}}%
{\end{array} \right.}

\newenvironment{sistemax}%
{\setlength\arraycolsep{0pt}
\left\lbrace \begin{array}{RCL}}%
{\end{array} \right.}

\begin{document}

\begin{equation*}
\begin{sistema}
\TTR_{\PLATEAU} = \frac{ir}{R_a} \\
k = \frac{R_a}{Q}
\end{sistema}
\end{equation*}

\begin{equation*}
\begin{sistemax}
\TTR_{\PLATEAU} &=& \frac{ir}{R_a} \\[2.5ex] % <-- need extra space
k &=& \frac{R_a}{Q}
\end{sistemax}
\end{equation*}

\end{document}

• Who down-voted both the query as well as both my answer and the one by @Cragfelt, all within the space of a few second? And, why? Care to enlighten us?
– Mico
Dec 3 '17 at 9:45
• Is there any way to know who downvoted our answers and the question? Dec 5 '17 at 10:08
• @Cragfelt - Only the admins would know who the downvoter is. I left a note with the site admins on Sunday evening, pointing out that the query as well as two of the three answers got downvoted, all within a minute of each other. (Bernard's answer was spared, I suppose, because he didn't post it until after the downvoting vandal had done his/her "work".) I also asked the admins to look into whether this was the work of a serial downvoter and, if so, to take appropriate countermeasures. I haven't heard back from them yet, though.
– Mico
Dec 5 '17 at 10:14

Needless to define a new environment: the dcases environment from mathtools (which loads amsmath) does the job.

Three other possibilities for the alignment of the = sign in the system (personally, I find the first solution nicer)

\documentclass{article}

\usepackage{empheq}
\usepackage{delarray}

\newcommand{\TTR}{\textit{TTR}}

\begin{document}

\begin{equation*}
\TTR(t) = \TTR_{\textrm{\tiny PLATEAU}} \bigl( 1 - e^{- k \, t} \bigr) \quad \text{con} \enspace \begin{dcases}
\TTR_{\textrm{\tiny PLATEAU}} = \frac{ir}{R_a} \\
k = \frac{R_a}{Q}
\end{dcases}
\end{equation*}
\bigskip
\begin{empheq}[left = {\TTR(t) = \TTR_{\textrm{\tiny PLATEAU}} \bigl( 1 - e^{- k \, t} \bigr) \quad \text{con} \enspace\empheqlbrace}]{align*}
\TTR_{\textrm{\tiny PLATEAU}} & = \smash[t]{\frac{ir}{R_a}} \\[0.5ex]
k & = \frac{R_a}{Q}
\end{empheq}
\bigskip

\begin{equation*}
\TTR(t) = \TTR_{\textrm{\tiny PLATEAU}} \bigl( 1 - e^{- k \, t} \bigr) \quad \text{con} \enspace \begin{dcases}\begin{aligned}
\TTR_{\textrm{\tiny PLATEAU}} & = \frac{ir}{R_a} \\[0.5ex]
k & = \frac{R_a}{Q}
\end{aligned}
\end{dcases}
\end{equation*}
\bigskip

\begin{equation*}
\TTR(t) = \TTR_{\textrm{\tiny PLATEAU}} \bigl( 1 - e^{- k \, t} \bigr) \quad \text{con} \enspace \begin{array}\{{@{}r@{{}={}}l@{}}.
\TTR_{\textrm{\tiny PLATEAU}} & \dfrac{ir}{R_a} \\[2ex]
k & \dfrac{R_a}{Q}
\end{array}
\end{equation*}

\end{document}


• I think (though I'm not entirely sure) that one of the OP's objectives was to align the two = symbols inside the "sistema". Any suggestions for how to achieve this within the dcases framework? :-)
– Mico
Dec 3 '17 at 13:45
• Hello @Mico, my goal is to align = symbols inside system of equations. Is possible to do it with your code? Dec 3 '17 at 17:02
• @GennaroArguzzi -- Does the second form, "sistemax", help achieve your goal?
– Mico
Dec 3 '17 at 17:49
• @Mico Yes, it's the right way. Dec 3 '17 at 18:20
• @GennaroArguzzi: I've added 3 other solutions, with alignment of the equal signs. Dec 3 '17 at 19:44

You need to add parameter r in the definition of your environment {r@{}l@{}}}.

\documentclass{article}

\usepackage{amsmath}

\newenvironment{sistema}%
{\end{array}\right.}

\begin{document}

\begin{equation*}
TTR(t) = TTR_{PLATEAU} \left( 1 - e^{- k \, t} \right) \text{ con }
\begin{sistema}
TTR_{PLATEAU} = \dfrac{ir}{R_a} \\[3ex]
k = \dfrac{R_a}{Q}
\end{sistema}
\end{equation*}

\end{document}


1. I replaced the long code \hspace{0.5cm} con \hspace{0.2cm} with \text{ con },
2. put [3ex] after \\ to add extra vertical space between lines, and
3. changed \frac to \dfrac (displaystyle fraction).
According to @barbara beeton's comment: "the spacing before the = sign in the image looks incorrect. are you sure that was produced with the & before the = sign?", there was a bad space produced by & so I removed it.
• the spacing before the = sign in the image looks incorrect. are you sure that was produced with the & before the = sign? Dec 4 '17 at 17:42