5 Quadrilateri ESERCIZI 125 Se sui lati di un angolo di vertice P si considerano due punti A e B e sull altro lato due punti C e D, in modo tale che AC e BD siano paralleli, e si conduce da D una parallela a BC che intersechi in E la retta PA, allora il segmento PB è medio proporzionale tra PA e PE. 126 In un triangolo ABC sia AM la mediana relativa al lato BC e sia O il punto medio di AM. Si consideri la semi- retta di origine C passante per O e sia K la sua intersezione con il lato AB; allora AK è un terzo di AB. 127 INVALSI 2014 Indica se ciascuna delle seguenti affermazioni è vera (V) o falsa (F). a. Condizione necessaria affinché un quadrilatero abbia le diagonali uguali è che sia un rettangolo b. Condizione sufficiente affinché un quadrilatero abbia le diagonali uguali è che sia un rettangolo c. Condizione necessaria e sufficiente affinché un rombo sia un quadrato è che abbia le diagonali uguali V F V F V F PRACTICE WITH CLIL The honeybees geometry Among the geometric patterns occurring in nature, honeycomb is certainly one of the most interesting. Bees build their honeycomb by excreting wax by eight wax-producing glands situated in their abdominal segments. The produced wax is liquid but it almost immediately becomes solid. The thickened wax is then collected, moistened and shaped; to produce wax, each bee burns large quantities of sugar and therefore, it is important that the produced wax is used so to optimize maximum results. The individual cell has the cross-section of a regular hexagon, a convex polygon made of six equal length sides and all interior angles of 120 degrees. Which theorem makes us state that a polygon with 6 sides has interior angles equal to 4 straight angles? This pattern makes it possible that each wall can be used for dividing two cells. However, why exactly the hexagonal pattern? Honeybees could have chosen others, like, for instance, those in the following pictures: Only three kinds of regular geometric shapes can be fitted to build honeycomb cells: the triangle, the square and the hexagon. By the way among them, a calculation can prove it, the cell perimeter of the hexagon is the least, but with equal area. Thus, if the hexagon shape is chosen to build the cells, the honeybees will use less wax. In mathematics, a problem of economic nature is often translated in a minimum problem: how to achieve best results doing less effort? The honeybees seem to know the answer. Put it into practice to justify the statement. Theorem no.23 affirms that The sum of the interior angles of a polygon with n sides is congruent to n 2 straight angles therefore in the case of a hexagon n = 6 sides n 2 = 4 straight angles hence each 2 angle equals __ of a straight angle. 3 221