The sliding argument If we move a right triangle in the plane in a direction parallel to one of its legs then the area swept by the triangle is equal to the area left behind. By ‘area swept’ (AS) we mean the set of points in the plane which the triangle occupies during the translation, but not including the initial position. Analogously, by ‘area left behind’ (AL) we mean the set of points in the plane that the triangle occupies during the translation, but not including the final position. Of course, the sliding argument reduces to the fact that parallelograms which are on the same base and between the same parallels have equal area, but we prefer to interpret it as a consequence of the area conservation of the triangle, as we shall explain now. In Figure 1(a), a triangle with vertices and is translated through a vector . Let be the area of the triangle before the translation, and