1. Open your sketchpad document.

2. Select the Selection Arrow Tool.


Click anywhere on the document to make sure everything is deselected.

3. Select the left endpoint of the triangle base AND one parallel line. From the menu bar select
> Construct
> Perpendicular Line

You have a line passing through the base endpoint that is perpendicular to the parallel lines.

4. Create the height segment.
Click anywhere on the document to deselect all objects.
Click on the intersection of perpendicular line and the top parallel line to create the point of intersect.
Select the segment tool.
Create a segment from the bottom of the left endpoint and the top intersection point. Make the segment thick and bright green.
Finally, select the perpendicular line and hide it.
> Display
> Hide line