@article{AM2_1982__24_136929,
     author = {C.E. Carroll},
     title = {Proof of {Oppenheim's} area inequalities for triangles and quadrilaterals.},
     journal = {Aequationes mathematicae},
     pages = {97--109},
     publisher = {mathdoc},
     volume = {24},
     year = {1982},
     zbl = {0511.51020},
     url = {http://geodesic.mathdoc.fr/item/AM2_1982__24_136929/}
}