Aligned inside cases not behaving as expected

Question

I wish to format a system of equations. I use:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$$\begin{cases}
\begin{aligned}
q_1 S_1^1 &+ q_2 S_2^1 &+ ... &+ q_n S_n^1 &= (1+r) S_0^1, \\
q_1 &+ q_2 &+ ... &+ q_n &= 1.
\end{aligned}
\end{cases}$$
\end{document}

I expect the q's and the equality sign to be below each other, but instead there is an indent in front of the q_1 and the =1 is totally on the right. How do I obtain my desired behavior?

Answer 1

1. You shouldn't use $$ in LaTeX.
2. You don't want cases, which adds unwanted horizontal space at its right if its contents isn't a set of rows of the form <formula>&<condition>.
3. You don't want aligned, which adds space between the columns.

With alignedat:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
[
\left{
\begin{alignedat}{5}
 & q_1 S_1^1 &&+ q_2 S_2^1 &&+ \dotsb &&+ q_n S_n^1 &&= (1+r) S_0^1, \
 &       q_1 &&+ q_2       &&+ \dotsb &&+ q_n       &&= 1.
\end{alignedat}
\right.
]
\end{document}

Answer 2

I don't think the aligned and cases machineries are all that well suited to handle the present job. I suggest you use an array environment instead.

The ^{} particles in row 1 assure that the subscripts associated with q and S^1 are placed at the same depth.

\documentclass{article}
\usepackage{array} % for '\newcolumntype' macro
\newcolumntype{C}{>{{}}c<{{}}} % col. type for binary and relational operators
\newcolumntype{L}{>{\displaystyle}l}
\begin{document}
[
\setlength\arraycolsep{0pt}
\renewcommand\arraystretch{1.33}
\left{
\begin{array}{ L *{4}{CL} }
q^{}_1 S_1^1 &+& q^{}_2 S_2^1 &+& \cdots &+& q^{}_n S_n^1 &=& (1+r)S_0^1 \
q_1 &+& q_2  &+& \cdots &+& q_n &=& 1 ,.
\end{array}
\right.
]
\end{document}

Answer 3

I assume that you want the q_1 S_1^1 and q_1 right-aligned, because that's what your given code outputs.

I don't understand why, but the alignment of the rightmost column comes closer to what you want if you put an extra & before the &=:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$$\begin{cases}
\begin{aligned}
q_1 S_1^1 &+ q_2 S_2^1 &+ ... &+ q_n S_n^1 &&= (1+r) S_0^1, \\
q_1 &+ q_2 &+ ... &+ q_n &&= 1.
\end{aligned}
\end{cases}$$
\end{document}

It seems to me that align treats the entries per row in groups of two or three, and also puts extra space after each triple (between the handwritten lines).