Cases environment left alignment

Question

As I understand, cases environment is centered by default: How to make it left aligned?

Answer 1

If you only need to left-align the two cases you can use align for this purpose.

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align*}
& \begin{cases}
     C=C_0+(Y-T)*\textit{MPC} \\
     Im=Im_0+\textit{MPM}*Y \\
     90+C=C_0+\textit{MPC}*\biggl(Y+\dfrac{20}{1-\textit{MPC}+\textit{MPM}}\biggr)-T \\
     10+Im=Im_0+\textit{MPM}*\biggl(Y+\dfrac{20}{1-\textit{MPC}+\textit{MPM}}\biggr)
   \end{cases} \\
& \begin{cases}
     90=Y+\dfrac{20*\textit{MPC}}{1-\textit{MPC}+\textit{MPM}} \\
     \\
     10=Y+\dfrac{20*\textit{MPC}}{1-\textit{MPC}+\textit{MPM}} \\
   \end{cases}         
\end{align*}

\end{document}  

Output:

If you also want to left-align all math stuff to the left margin of the page, you have to load the document class with the option fleqn, in this case

\documentclass[fleqn]{article}

and add the line

\setlength{\mathindent}{0pt}

in the preamble.