# System with two equations - need brackets

I want to make a system with two equations but with biggest numbers, because now I can't see the number in the paper and my teacher will not see well. The system is:

\documentclass{article}
\usepackage[top=2cm, bottom=2cm, left=2cm, right=2cm]{geometry}
\usepackage{amstext}
\begin{document}
$$\left\{ \begin{matrix} I_1=I_2+I_3+I_4\\ I_3=I_5+I_6\\ \end{matrix}\right. \Leftrightarrow \left\{ \begin{matrix} \frac{5-u_A}{1\times10^3}=\frac{u_A-4}{2,2\times 10^3}+ \frac{u_A-u_B}{4,7\times 10^3}+\frac{u_A}{4,5\times 10^3}\\ \frac{u_A-u_B}{4,7\times 10^3}=\frac{u_B}{6,8\times10^3}+2\times10^3 \end{matrix}\right.$$
\end{document}

• Please post complete code. $$ ought not to be used in LaTeX. It is 20+ years deprecated. – cfr Oct 5, 2016 at 2:16 • Load amsmath and use \dfrac or \displaystyle\frac. – Werner Oct 5, 2016 at 2:45 • @cfr If I can't use $$what can I use for the same function? Oct 5, 2016 at 8:33
• @CarmenGonzález See Why is $…$ preferable to ? Oct 5, 2016 at 8:40

Something like this? Note the use of (a) the array package and array environments, (b) the use of (typographic) struts so that the rows in the first array line up with the ones in the second array, (c) curly braces to encase the commas that serve as decimal markers and (d) \addlinespace to provide a bit more whitespace between the rows.

\documentclass{article}
\usepackage{array,amssymb,booktabs}
\newcommand{\tallstrut}{\vphantom{\frac{5_A}{4,10^3}}} % a typographic strut

\begin{document}
$\left\{ \begin{array}{>{\displaystyle\tallstrut}l@{}} I_1=I_2+I_3+I_4\\ \addlinespace I_3=I_5+I_6 \end{array} \right. \Leftrightarrow \left\{ \begin{array}{>{\displaystyle}l@{}} \frac{5-u_A}{1{,}0\times10^3}=\frac{u_A-4}{2{,}2\times 10^3}+ \frac{u_A-u_B}{4{,}7\times 10^3}+\frac{u_A}{4{,}5\times 10^3}\\ \addlinespace \frac{u_A-u_B}{4,7\times 10^3}=\frac{u_B}{6{,}8\times10^3}+2\times10^3 \end{array} \right.$

\end{document}


Addendum: As @egreg has noted in a comment, the same output may be achieved -- with slightly less typing -- by employing dcases environments (provided by the mathtools package):

\documentclass{article}
\usepackage{amssymb,booktabs,mathtools}
\newcommand{\tallstrut}{\vphantom{\frac{5_A}{4,10^3}}} % a typographic strut

\begin{document}

$\begin{dcases} I_1=I_2+I_3+I_4\tallstrut\\ \addlinespace I_3=I_5+I_6\tallstrut \end{dcases} \Leftrightarrow \begin{dcases} \frac{5-u_A}{1{,}0\times10^3}=\frac{u_A-4}{2{,}2\times 10^3}+ \frac{u_A-u_B}{4{,}7\times 10^3}+\frac{u_A}{4{,}5\times 10^3}\\ \addlinespace \frac{u_A-u_B}{4,7\times 10^3}=\frac{u_B}{6{,}8\times10^3}+2\times10^3 \end{dcases}$

\end{document}

• It's works! I will use your code. Oct 5, 2016 at 8:34
• I'd give a try to dcases from mathtools instead of the complicated array construction. Oct 5, 2016 at 8:39
• @egreg - Thanks. I've provided an addendum to show how the same output may by achieved with dcases environments instead of array environments.
– Mico
Oct 5, 2016 at 9:00
• @Mico How I can put the brackets aligned at the left of the page in all lines? With the array environments. Oct 5, 2016 at 18:01
• @egreg How I can put the brackets aligned at the left of the page in all lines? With the array environments. Oct 5, 2016 at 18:06