EXPLANATION FILE OF PROGRAM GCEULER
                     ===================================

 The program gceuler in C++ or Pascal draws the Euler's circle of a triangle ABC.


 Procedure Draw_Circle3P
 -----------------------

 This procedure draws a circle passing through three given points,
 
   (x1,y1), (x2,y2) and (x3,y3).

 First we seek the cordinates of the circle center, (a,b) in equation:

   x² + y² -2ax -2by + c = 0                        (1)

 So we have to solve the following linear system:

   x1² + y1² -2ax1 -2by1 + c = 0
   x2² + y2² -2ax2 -2by2 + c = 0                    (2)
   x3² + y3² -2ax3 -2by3 + c = 0

 This is equivalent to the linear system:

   2(x2-x1) a + 2(y2-y1) b = x2² +y2² -x1² -y1²     (3)
   2(x3-x1) a + 2(y3-y1) b = x3² +y3² -x1² -y1² 	   

 This linear system (3) can be easily solved by procedure System2D (see below).

 Having (a,b), the coefficient c is given by any equation of system (2) and the radius r
 is given by: r² = a² + b² - c.

 Having a, b, r, we use procedure Circle of unit graph_2d to draw a circle in physical
 coordinates, knowing center (x,y) and radius, r and procedure CroixXY to draw a cross
 at center of the circle.

 Note: this procedure will fail if the 3 points are not distinct.


 Procedure System 2D
 ===================

 We have to solve the linear system:

    a  x + b  y = c
    a1 x + b1 y = c1

 The solution is:  y = (ca1-c1a)/(ba1-ab1)
                   x = (c-by)/a (if a<>0)
                or x = (c1-b1y)/a1 (if a=0).

 Note: this procedure will fail if (ba1-ab1) = 0.

 
 We also use procedure Line2D to determine coefficients a, b of y=ax+b straight line 
 passing through 2 points (x1,y1) and (x2,y2) and procedure Draw_Line to Draw line 
 y=ax+b from (x1,y1) to (x2,y2). These are used to draw the Euler's Straight Line
 passing through points H, G, O, O1 of triangle ABC.

 To draw the Euler's circle of triangle ABC, we choose the 3 middles of sides AB, BC 
 and AC and procedure Draw_Circle3P.


 Jean-Pierre Moreau, Paris.