### 26.13.1 Measurement and display

Many commands to find measures have a version ending in at (or atraw) which are used to interactively find and display the measure in a 2D geometry screen. To use them, open a geometry screen with Alt+G and select a measure from the Mode ▸ Measure menu. Now, clicking on the names of objects (or, if a point is being selected, a name will be automatically generated if clicking on an open point) with the mouse and then clicking on another point will put the measurement at that point; if the mode is the version ending in at resp. atraw, then the measurement will have a label resp. appear without a label.

The commands with at and atraw versions are the following:

- distance, distanceat, distanceatraw — This finds the distance between two points or other geometric objects (see Section 26.13.2).
- angle, angleat, angleatraw — This finds the measure of an angle BAC given points A, B and C (see Section 26.13.4).
- area, areaat, areaatraw — This finds the area of a circle or a polygon which is star-shaped with respect to its first vertex (see Section 26.13.6).
- perimeter, perimeterat, perimeteratraw — This finds the perimeter of a circle, circular arc or a polygon (see Section 26.13.7).
- slope, slopeat, slopeatraw — This finds the slope of a line, segment, or two points which determine a line (see Section 26.13.8).

These commands can also be used from the command line. They are like the measurement command but take an extra argument, the point to display the measurement. When using the version ending in at, use names for the objects rather to create the objects within the measurement command.

#### Examples

S:=square(1/4+i/4,1/2-i/2); areaat(S,8/7+i/4)

S:=square(1/4+i/4,1/2-i/2); areaatraw(S,8/7+i/4)

More sophisticated legends are created with the legend command (see Section 26.3.3).

S:=square(1/4+i/4,1/2-i/2); a:=area(S); legend(8/7+i/4,"Area(S)="+string(a),blue)

The extract_measure command displays a measurement.

- extract_measure takes atcommand, one of the at or atraw commands (which displays a measurement).
- extract_measure(atcommand) returns the measurement.

#### Example

A:=point(-1); B:=point(1+i); C:=point(i); extract_measure(angleat(A,B,C,0.2 i))

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