It is proved that the midpoints of four semi-circular arcs are connected to form a parallelogram or line segment by alternately making inner and outer semicircles with four sides of an arbitrary quadrilateral as the diameter (two semicircles can be made with one side of the quadrilateral as the diameter, and the semicircle near the diagonal intersection of the quadrilateral is the inner semicircle).

The upper right indicates that the letter represents the estimated value of the parameter.

For example, the univariate quadratic regression model y=a+b*x+u, where x is the explanatory variable, y is the explained variable and u is the error term. A and b represent two parameters of the model respectively. A and b are directly above each other, indicating the estimated values of parameters.

In mathematics and physics, radian is the unit of measurement of angle. It is a unit derived from the international system of units, abbreviated as rad. Definition: The central angle of an arc whose arc length is equal to the radius of a circle is 1 radian. According to the definition, the number of radians in a week is 2πr/r=2π, and the 360 degree angle =2π radians. So 1 radian is about 57.3, that is, 57 17' 44.806''.