\startcases

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There are several built-in commands for displaying mathematical formulas. For instance

\startcases, \stopcases
\startmathmatrix, \stopmathmatrix
\startalign,\stopalign

Each has its own advantage and can be customized: for a thorough understanding of these commands please refer to Aditya Mohajan's excellent user's guide Using \startalign and friends. These commands can be found in core-mat.tex file, which contains most of the core ConTeXt math macros.

This is an example of aligned equations:
\startformula
 \startcases[align={right,left},distance=3pt]
   \NC 2x + 3y \NC{} = 10 \NR
   \NC 3x + 2y \NC{} = 5,   \NR
 \stopcases
\stopformula
and this is another one:
\startformula
 \startmatrix[align={right,left},distance=3pt,left={\left\{}, right={\right.}]
   \NC 2x + 3y \NC{} = 10 \NR
   \NC 3x + 2y \NC{} = 5   \NR
 \stopmatrix
\stopformula

In these structures it is important (well, in the above example it is not important and the result is the same, but I have been told that it is necessary in some other situations...) to notice that the sign "=" is preceded by "\NC{}, since otherwise that sign would be considered as a binary operator and the alignment would be disturbed (at least this is what I guess…Otared K.).

One of the powerful structures in displaying mathematical formulas is the possibility of defining custom maths alignment. For instance the following definition

\definemathmatrix[alignedcases]
		[align={right,left,left},
		distance=3pt,
		left={\left\{},
		right={\right.},
		style=\displaystyle]

creates two new commands,

\startalignedcases, \stopalignedcases:

after which one may use them in a displayed formula as follows:

The following formula is displayed in two lines with an appropriate alignment of the equations:

\startformula
\startalignedcases
 \NC -\Delta u + g(u) \NC{} = f \NC \quad\mbox{in } \Omega\NR
 \NC {\partial u \over \partial {\bf n} } \NC{} = h(u) \NC \quad\mbox{on } \partial\Omega   \NR
\stopalignedcases
\stopformula