Four-point * * * circle: A quadrilateral composed of four points can be diagonally complementary. And vice versa, that is, the quadrangles of four-point * * * circles are diagonally complementary.

Slope formula: The coordinates of two points on a straight line are A (X 1, Y 1) and B (X2, Y2) respectively, so the slope of straight line AB is: k=(y 1-y2)/(x 1-x2).

You can also directly use the coordinates of these two points to describe the analytical formula of a straight line: (x-x1)/(x1-x2) = (y-y1)/(y1-y2), where x and y are any points on the straight line. After simplifying this equation, Y 1), then use the point inclination method to find the analytical formula of the straight line: y-y 1=k(x-x 1), and then simplify.

Distance formula from point to straight line: let the analytical formula of straight line be y=kx+b (this is a general form), which should be simplified to Ax+By+C=0, and the point outside the straight line is A(x 1, y 1), then the distance from a point to straight line can be expressed as d = (ax1).