This page shows how to construct a perpendicular at the end of a ray with compass and straightedge or ruler. This construction works as a result of Thales Theorem.
From this theorem, we know that a
of a circle always
a right angle to any point on the circle, so by using it in reverse we produce a right angle.
Printable step-by-step instructions
The above animation is available as a
printable step-by-step instruction sheet, which can be used for making handouts
or when a computer is not available.
Why it works
This construction works as a result of
From this theorem, we know that a diameter of a circle always subtends a right angle to any point on the circle.
In this construction we:
- Create a circle that has the end part of the given ray as a chord. (step 3).
- We then draw a diameter of the circle (step 4).
- When we close the triangle in step 5, we know it must be a right triangle from Thales Theorem.
The image below is the final drawing from the animation above.
||AB is the diameter of the circle center D.
|| A diameter is a line through the center of a circle.
||m∠APB = 90°
||From Thales Theorem - the diameter of a circle
to any point of the circle's circumference - here P.
||AP is perpendicular to the endpoint of the ray PB
Try it yourself
Click here for a printable worksheet
containing two problems to try.
When you get to the page, use the browser print command to print as many as you wish. The printed output is not copyright.
Other constructions pages on this site
Circles, Arcs and Ellipses
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