Math & Science ⇒ Alignement problem in exam class

[Cham]

I'm having some troubles in aligning some simple equations. Here's the MWE code to explain my issue, and a preview :

Code: Select all

\documentclass[11pt]{exam}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage[margin=1in]{geometry}

\begin{document}
\begin{questions}
\question[10] Some equations to solve :

\medskip

\begin{parts}
	\part
		\setlength{\abovedisplayskip}{0pt}
		\begin{flalign*}
			5 x + 2 y &= 3 y, & \\
			(x + y) + 2 x &= 4 x + y.
		\end{flalign*}
	\vfill
	\part
		\begin{flalign*}
			x + 2 y &= 3 y, & \\
			2 x + y &= 2.
		\end{flalign*}
	\vfill
	\part
		\begin{flalign*}
			x + y &= 5, & \\
			y &= 1.
		\end{flalign*}
	\vfill
\end{parts}

\end{questions}
\end{document}

Preview :

2equs.jpg (12.89 KiB) Viewed 3450 times

I need the equations sets to be aligned with their left letter, and on the left side of the page. Currently, the flalign command doesn't seem to work. How to fix that ?

[Cham]

Weird ! Adding & at the end of the first equation line solves the left flushing issue. Why ?

Code: Select all

\documentclass[11pt]{exam}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage[margin=1in]{geometry}

\begin{document}
\begin{questions}
\question[10] Some equations to solve :

\begin{parts}
	\part
		\begin{flalign*}
			5 x + 2 y &= 3 y, & \\
			(x + y) + 2 x &= 4 x + y.
		\end{flalign*}
	\vfill
	\part
		\begin{flalign*}
			x + 2 y &= 3 y, & \\
			2 x + y &= 2.
		\end{flalign*}
	\vfill
	\part
		\begin{flalign*}
			x + y &= 5, & \\
			y &= 1.
		\end{flalign*}
	\vfill
\end{parts}

\end{questions}
\end{document}

Now, how to make the set of equations to be centered on its left letter ?

[Cham]

I've found an hack, but I don't think this is a good way of doing this. Is there a "local" way of reducing the vertical space between the text and the aligned equations ? I don't want the command to change everything in the whole document :

Code: Select all

\documentclass[11pt]{exam}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage[margin=1in]{geometry}

\begin{document}
\begin{questions}
\question[10] Some equations to solve :

\medskip

\begin{parts}
	\part
		\setlength{\abovedisplayskip}{0pt} % <=== stupid hack ?
		\begin{flalign*}
			5 x + 2 y &= 3 y, & \\
			(x + y) + 2 x &= 4 x + y.
		\end{flalign*}
	\vfill
	\part
		\begin{flalign*}
			x + 2 y &= 3 y, & \\
			2 x + y &= 2.
		\end{flalign*}
	\vfill
	\part
		\begin{flalign*}
			x + y &= 5, & \\
			y &= 1.
		\end{flalign*}
	\vfill
\end{parts}

\end{questions}
\end{document}