平角和直角

A straight angle is an angle whose measure is $180^\circ$. So given points $A$, $B$, and $C$ on a line, where $B$ is between $A$ and $C$, $\angle ABC$ is a straight angle.

A right angle is an angle whose measure is $90^\circ$.

A straight angle is an angle with measure 180 degrees. It is called a straight angle because if there were an angle that had measure 180 degrees, the three points that form the angle would be in a straight line. A right angle is an angle with measure 90 degrees. It would be like the edge of a paper; in fact, the edge if a paper is 90 degrees!

a straight angle is formed by a line which measures 180 and a right angle is an angle which bisects the angle formed by 2 halves of straight line

You must think that an angle which measures exactly 90° is a right angle.... Suprisingly it is not true!! The angles like edges of a paper or a cuboid are some of the examples of right angle. It was later on calculated that 1 right angle =90°= 100 grades= π/2 radians. I don't know the real definition but I wrote this so as to clear the misconception about the topic.

The same case is with the straight angle. It called so beacause straight angle is as straight as an arrow and appears to be a line. 1 straight angle =180°=π radians=200 grades

In his book The Elements，Euclid defines a right angle as the angle formed when "a straight line standing on another line makes adjacent angles equal to one another". This is a cool definition because it doesn't involve any unit of measurement such as degrees or radians, so it is in some sense a universal definition. A straight angle is an angle formed by a line, which happens to be two right angles.