As shown below: Two sheep are fighting. Two sheep are facing the top corner.

From this, I thought of the vertex pair in mathematical terms. As shown in the figure below:

Inverted vertex angle: that is, if two sides of one angle are opposite extension lines of two sides of another angle, and these two angles have a common vertex, then these two angles are antipodal angles. The antipodal angle ranges between 0 degree and 180 degree, excluding 0 degree and 180 degree. The antipodal angle is two angles with special position, and the equality of antipodal angle reflects the relationship between the two angles.