@article{TM2_2011_XIV_1_a3,
     author = {Borislav Lazarov},
     title = {Teaching envelopes in secondary school},
     journal = {The Teaching of Mathematics},
     pages = {45 },
     publisher = {mathdoc},
     volume = {XIV},
     number = {1},
     year = {2011},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TM2_2011_XIV_1_a3/}
}