Today’s random geometric construction is the perpendicular line from an end point. I like this one a lot because it utilises a circle theorem.
First, draw your arbitrary line, with an arbitrary dot, ideally near the end point of the line like so:
Next, construct a circle with its center as the dot, and radius as dot-to-edge:
Next draw a diameter from point of intersection furthest from your line endpoint:
And finally, join the diameter to your line endpoint:
The angle in a semi-circle is always 90 degrees. Job done! All that’s left is to colour it in, because colouring maths diagrams is underrated.