题目

如图（1），在三角形中，，若，则；若类比该命题，如图（2），三棱锥中，面，若点在三角形所在平面内的射影为，则有什么结论？命题是否是真命题．    答案：解析：命题是：三棱锥中，面，若点在三角形所在平面内的射影为，则有是一个真命题．证明如下：在图（2）中，连结，并延长交于，连结，则有．因为面，，所以．又，所以．于是．动词应用 （共10 小题，计10分）用动词的适当形式填空，必要时可加助动词或情态动词。1.I think you can’t put off (go)to see the dentist any longer.2.The teacher told the boy _________ (not play) football on the street.3.You could _________(give) him a ticket to a ball game.4.Your mother like a color bike. Why don’t you (buy) one for her.5.I will write to you as soon as I _________(get) home.6.His father watched TV while his mother __________(cook).7.It’s difficult ________(find) a rabbit in so many holes.8.—What were you doing when he arrived?—I ________(write) a letter at that time.9. _________(copy) others’ homework is wrong.10.When I don’t know what _______(do), I usually ask him for help.