Applying this method we evaluate (see Fig. 1) the projections of the vector (the height at P 2 of the given triangle), and also the co-ordinates of the point in which the perpendicular form

the origin meets the plane of the triangle. In this way, as will be shown, we find immediately the co-ordinates of the perpendicular projection of the given point P 2(x 2, y 2, z 2) on the straight line passing through P 3 and P 1, and also the equation of the plane of the triangle.