A Flat Angle is one that is formed when the direction of the vector changes to the opposite, that is, the orientation is completely opposite. To draw a fully described straight angle it is necessary that the two vectors form a single line but that their directions point to the opposite sides. Placing the point of the compass in the middle of both vectors and drawing a 180 degree semicircle creates a non-existent or flat angle between the two vectors. It is called a plane because it is perpendicular to the line formed by the opposite vectors, when drawing, it is not even perceived because it is immersed in the plane.
Its application in practice is quite simple, in fact, the protractor, which is usually a semicircular ruler, indicates in its smooth part that the flat angle should always be there in the drawing, its shape is similar to the one we see in the initial image.
In everyday life, when saying that a person or a vehicle made a 180 degree turn, it is making a turn with a resulting flat angle regardless of the size of the axis on which it is made, for example, the island of an avenue.
Mathematically, a straight angle is worth pi rad, that is, 180 ° times one radian. To represent a straight angle, just draw a semicircle and denote that the union of the diameter is a straight angle as well. It can be done with a compass or with a protractor, a straight rule should also be used.
