Definition:Right Angle
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Definition
A right angle is an angles that is equal to half of a straight angle.
The measurement of a right angle is $90^\circ$ or $\dfrac \pi 2$ radians.
Perpendicular
As Euclid defined it:
- When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right, and the straight line standing on the other is called a perpendicular to that on which it stands.
(The Elements: Book I: Definition $10$)
In the above diagram, the line $CD$ has been constructed so as to be a perpendicular to the line $AB$.
