# Knowledge Base

## Solution 10048: Calculating the Arc Measure for an Arc Created by a Chord, Tangent Lines, and Secant Lines in Cabri Geometry™ II.

### How can I calculate the arc measure for an arc created by a chord, tangent lines, and secant lines in Cabri Geometry II?

To calculate the measure of arcs in Cabri Geometry II, please follow the instructions listed below.

Calculating the arc measure for an arc created by a chord:

• Use the circle tool to create the circle with center point O.
• Use the point on object tool to define two points, A and B. Use the line segment tool to construct a segment between them. This is the chord AB.
• The measure of the arc AB is equal to the angle measure from AOB. Use the angle measure tool to measure the angle of AOB. Remember to select the points in that order, A then O then B.

Calculating the arc measure created by two secant lines:

• Use the circle tool to create the circle with center point O.
• Use the point on object tool to define two points, A and B. These are the points where the secant lines will touch the circle.
• Use the point tool to create a point C anywhere inside or outside the circle (but not on the circle).
• Use the Line tool to plot lines connecting AC and BC. These are the secant lines.
• The measure of the arc AB is equal to the angle measure from AOB. Use the angle measure tool to measure the angle of AOB. Remember to select the points in that order, A then O then B.

Calculating the arc measure created by two tangent lines:

• Use the circle tool to create the circle with center point O.
• Use the point on object to create two points, A and B. These will be the points where the tangent lines intersect the circle.
• Use the line segment tool to create two segments, between AO and BO.
• Use the perpendicular line tool to create lines at points A and B, perpendicular to AO and BO respectively. These are the tangent lines.
• The measure of the arc AB is equal to the angle measure from AOB. Use the angle measure tool to measure the angle of AOB. Remember to select the points in that order, A then O then B.

NOTE: Cabri seems to only measure the minor arc. To get the angle measure of the major arc, subtract the angle measure of the minor arc from 360. This can be done in the software using the Calculate tool.