Objects creation and modification	Drawing geometric objects
Drawing lines
Overview     Operations     Commands

How to draw a straight line perpendicular to a curve and tangent to an other one

1. Click on the menu Draw  Line....  Perpendicular, tangent
2. Specify the first vertex of the line on the the object, near the perpendicular.
   

   A Deferred Perpendicular Snap will help you to select the point.

3. Moving the mouse cursor you can now see the first vertex to move in order to keep the line perpendicular to the object.
4. Specify the second vertex of the line on the the object, using the Tangent Snap.

The action is completed using a Deferred Perpendicular Snap for the first requested point, see also Using the Object Snaps

 Command line: LINE_PT