题目

如图是一体积为72的正四面体，连结两个面的重心E、F，则线段EF的长是_____________. 答案：.解析：设棱长为a,则高h=a.V=×a2·a=a3=72，∴a=6,EF=×a=2.A Simple Scientific ExperimentBelow is a description of a single scientific experiment. It shows us how iron reacts ( 反应) with air and with water.Aim : To find out if iron rusts( 生锈) (a) in dry air ; (b) in water that has no air in it ( air- free water) ; (c) in ordinary water.Materials : 3 clean iron nails ( 铁钉) ; test tubes ; test tube holder ; cotton wool ; oil ; Bunsen burner.Iron in dry airMethod(1)Put some iron nails at the bottom of a test tube.(2)Push some cotton wool down the tube.(3)Leave the tube for one week.ResultAfter one week, the nails have not rusted.ConclusionIron does not rust in dry air.Iron in air-free water Method(1)Half-fill a test tube with water.(2)Boil the water for three minutes. (This makes sure there is no air in the water.(3)Put two or three clean nails in the water.(4)Add some oil to the water. (This will keep air out of the water.)(5) Leave the tube for one week.Result The nails do not rust in the tube with air-free water.ConclusionIron does not rust in air- free water.Iron in ordinary waterMethod (1)Half –fill a test tube with water and add two or three clean nails.(2)Leave the tube for one week.ResultThe nails rust in the tube with ordinary water.ConclusionIron rusts in ordinary water. (238 words)小题1:In the first part of the experiment the cotton wool is most probably used to ______________.A．keep the air dryB．keep the tube wetC．make the nails rustD．keep the tube clean小题2:What are necessary steps to make sure the water in the tube is air free?①Leave half a tube of water in it              ②Boil the water for some time③Put some clean iron nails in the tube           ④Add some oil to the waterA．①②B．①③C．②③D．②④小题3:We can conclude from the experiment that only ____________ makes iron rusty.A．dry airB．cotton woolC．ordinary waterD．air-free water