Multiline equations

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Be sure to also read Using \startalign and friends written by Aditya Mahajan.

Basic Alignment with \startalign

\startformula\startalign
 \NC v \NC = u + at \NR
 \NC h \NC = ut + \frac12 gt^2 \NR
\stopalign\stopformula

\NC stands for new column, \NR stands for new row.

Changing the number of columns

The above equations were aligned at =. Suppose you also want the + to align. Well, this is simple in context, simply specify the number of columns with \startalign

\startformula \startalign[n=3]
 \NC v \NC = u  \NC + at \NR
 \NC h \NC = ut \NC + \frac12 gt^2 \NR
\stopalign \stopformula

Equation numbering with \startplaceformula

Aligned equations can be numbered by using \startplaceformula (as usual), and by placing a tag after \NR

\setuplayout[scale=0.8,width=13cm]
\startplaceformula
\startformula \startalign
 \NC v \NC = u + at \NR[eq:v]
 \NC h \NC = ut + \frac12 gt^2 \NR[eq:h]
\stopalign \stopformula 
\stopplaceformula
Equation~(\in[eq:v]) tells the final velocity after time~$t$ and equation~(\in[eq:h]) tells the distance travelled in time~$t$.

Sub-numbering with \startsubformulas

The numbering can be changed to a subformula style by encapsulating \startplaceformula with \startsubformulas :

\setuplayout[scale=0.8,width=13cm]
\startsubformulas[eq:total]
\startplaceformula
\startformula \startalign
 \NC v \NC = u + at \NR[eq:v]
 \NC h \NC = ut + \frac12 gt^2 \NR[eq:h]
\stopalign \stopformula 
\stopplaceformula
\stopsubformulas 

In~(\in[eq:total]), equation~(\in[eq:v]) tells the final velocity after time~$t$ and equation (\in[eq:h]) tells the distance travelled in time~$t$.

Specifying and defining alignment with \definemathalignment

If you want more control over the formatting, and want the middle column to be center aligned, you can do that by

\startformula \startalign[n=3,align={1:right,2:middle,3:left}]
 \NC v \NC = u \NC+ at \NR
 \NC h \NC= ut \NC+ \frac12 gt^2 \NR
\stopalign \stopformula

This mechanism allows fancier alignments like

\startformula \startalign[n=4,align={1:right,2:right,3:middle,4:left}]
 \NC \text{We have} \quad \NC v \NC = u \NC+ at \NR
 \NC \text{and}     \quad \NC h \NC= ut \NC+ \frac12 gt^2 \NR
\stopalign \stopformula

These kind of new alignment can be defined using \definemathalignment.

\definemathalignment
    [demom]
    [n=4,align={1:right,2:right,3:middle,4:left}]

\startformula \startdemom
 \NC \text{We have} \quad \NC v \NC = u \NC+ at \NR
 \NC \text{and}     \quad \NC h \NC= ut \NC+ \frac12 gt^2 \NR
\stopdemom \stopformula

A second example, to emulate gather environment of amsmath, we can use

\definemathalignment
    [gather]
    [n=1,align={1:middle}]

\startformula \startgather
 \NC  ax^2 + bx + c = 0 \NR
 \NC \text{roots} = \frac{ -b \pm \sqrt{b^2 - 4ac}}{2a} \NR
\stopgather \stopformula

Cases

Context provides a \startmathcases \stopmathcases pair to make it easy to get cases.

\startformula
\delta_{ij} =
 \startmathcases
 \NC 1 \NC \text{if } i = j \NR
 \NC 0 \NC \text{otherwise} \NR
 \stopmathcases
\stopformula

gives

The cases environment consists of two columns, separated by \NC. The second column is by default in math mode. To typeset the second column in text mode, use \TC

\startformula
\delta_{ij} =
 \startmathcases
 \NC 1 \TC if \m{i = j} \NR
 \NC 0 \TC otherwise \NR
 \stopmathcases
\stopformula

Each line must end with a \NR.