Given a △ABC with its area equal to 1, suppose that the vertices of quadrilateral P1P2P3P4 all lie on the sides of △ABC. Show that among the four triangles △P1P2P3,△P1P2P4,△P1P3P4,△P2P3P4 there is at least one whose area is not larger than 1/4. geometrygeometry unsolved