Quoting "fdu.xiaojf@gmail.com" :

Hi all,

I have two questions about math formula.

1) I'd like to use \startalign to typeset mathematical formulas since it is more convenient than \eqalign. But the formula number cannot be located between two lines just as \eqalign can. I hope following example can make me clear.

\starttext

\placeformula \startformula \eqalign{ a + b & = c + d\cr & = e + f\cr} \stopformula

\placeformula \startformula \startalign \NC a + b \NC = c + d \NR[+] \NC \NC = e + f \NR \stopalign \stopformula

\stoptext

The number of equations in second formula can only be put on the first line or second line but not between them.

My question is how to make the location of formula number in \startalign the same as it is in \eqalign ?

As of now, no. This is equivalent to what split environment in amsmath does, and right now, there is no context equivalent. It is easy to provide simple support, (actually even some of the more sophisticated features), and has been on my todo list for a long time now. You can get something by using matrix environments, but then the interline spacing is not correct. See http://dl.contextgarden.net/myway/mathalign.pdf page 21.

2) It seems that \left and \right cannot be used on separate groups. Here is a simple example:

\starttext

\placeformula \startformula \eqalign{ a & = \left(\frac{b+c}{d+e}\cr &\phantom{=~} + \frac{g}{h}\cr} \stopformula

\stoptext

If I change \left and \right to \big, it compiles successfully. But I think \left and \right are more convenient. Are there some solutions to use \left and \right in this situation ?

The easier way is to use manual scaling, for example use \big, \bigg, \Big, \Bigg or you can have a more fine-tuned control by \protected\def\domathbig{\@@dobig} and then use \domathbig{1.25} to have something which is 1.25 the body font size, etc. The default values are \def\big {\@@dobig{0.85}} \def\Big {\@@dobig{1.15}} \def\bigg{\@@dobig{1.45}} \def\Bigg{\@@dobig{1.75}} A more automated way is to use \vphantom, something like \eqalign{ a &= \left( \vphantom{} \right. & \left. \vphantom { } \right) } but this quickly becomes unmanageable if you have more than two expressions. A more sophisticated approach is taken by nath package in latex, see its documentation on CTAN. With nath it is as easy as a \wall = \left( \\ \right) \return There is a nath module for ConTeXt, but it is old and was made before Hans provided all the amsmath like alignment features. I do not know if you can still use it these days. HTH, Aditya