Geometrica
Bruno Autin, CERN, Switzerland
Geometrica versus Cabri or Sketchpad
Geometrica and Mathematica
Euclidean and Analytical Geometries
Primary concept in EUCLIDEAN geometry: POINT > superb reasoning (unfortunately disappearing!)
Primary concept in ANALYTICAL geometry: COORDINATES > unification of geometry and algebra (Descartes and the theory of curves)
Browsing Geometrica
The best navigation consists of choosing a simple problem and seeing how it is solved. We shall study the generation of a parabola as the envelope of one edge of a square ruler whose vertex describes a line and whose other edge passes through a fixed point.
Point, line, and first drawing
The straight line is the y axis.
Bound point and Euclidean line
The edge passing through m is defined by a second point bound to d.
Geometrical function
The other edge d_{2} is perpendicular in m_{1} to d_{1}.
Derivative of a geometrical function
When the parameter t varies, the line
Animation and story board
One can now recapitulate the construction by using the command Movie, which produces the six figures by integrating the figure n+1 to the nth one.
The movie is adapted to a demo, but to keep the trace of the construction, it is more appropriate to use StoryBoard, which puts the figures in a table.
Viewing the envelope process
We are now curious to see the motion of the line d_{2}. We then make a table of the graph of the line for a whole range of t.
Study of a conic
To find the equation of the envelope, we elminate t between the coordinates of m_{p}.
Geometrica can recognize the nature of the conic from its Cartesian equation r x y + t
and determine its focus and directrix.
Summary
New Functions
Parallel and paraxial lines
Walls as examples of parallel polygons
A line L' is parallel to a line L when it is generated by the end of a segment attached and normal to L. This defintion assumes that L represents a continuous and derivable function, which is not true for a polygon that has no normal at a vertex. A convention is introduced in Geometica to define a parallel polygon. It is illustrated for the walls of a house.
The wall has been defined with openings that can be seen if the option Ribbon is used. The inner polygons of the "ribbon" can be painted to distinguish the full and empty parts of the wall.
The drawing can finally be completed with the dimensions of the wall.
Paraxial rays in optics
Paraxial lines are generated in the same way as parallel lines, but the distance between the lines varies according to a prescribed function.
This is a regular motion, according to the amplitude of the oscillation; the paraxial line may take fancier shapes.
Custom composite objects
