How to draw the Altitude On a Exterior Angle Property

This is done because the side may not be long enough to perform the steps that follow.

2.  Set the compass on the opposite vertex (here R) and set the width to beyond the line PQ.

3.  Make two arcs across PQ, creating the points A and B.

4.  From A and B, make two arcs that overlap, creating point C. Make sure both arcs are drawn with the same compass width.

5.  Draw a line through R and C. Label the point S where it crosses PQ.

The line segment RS is the external altitude of the triangle PQR.