题目

如图所示，在正方体ABCD—A1B1C1D1中，E，F分别为CC1，AA1的中点，画出平面BED1F与平面ABCD的交线. 答案：【解析】在平面AA1D1D内，延长D1F， ∵D1F与DA不平行， ∴D1F与DA必相交于一点，设为P， 则P∈D1F，P∈DA. 又∵D1F⊂平面BED1F，AD⊂平面ABCD， ∴P∈平面BED1F，P∈平面ABCD. 又B为平面ABCD与平面BED1F的公共点，连接PB， ∴PB即为平面BED1F与平面ABCD的交线.如图所示. You probably know about Single's Day(Nov.11) in China, when people stay up very late at night to shop online in order to【1】(节省) a lot of money. Well, the US has something【2】(相似的) called Black Friday.Black Friday is the Friday after Thanksgiving. This year it will【3】(落) on Nov.27. It's the start of the holiday shopping【4】(季) and most people don't have to go to work that day. So people start to think about what【5】(礼物) they want to buy for Christmas.The name Black Friday comes from ink. A long time ago, shops used ink for book keeping all their sales. So when there was a lot of black ink on the page that meant the business sold a lot.While Single's Day is usually for online shopping, a lot of shopping on Black Friday is done at actual stores. Shoppers look for deals especially on electronics【6】(像) televisions, computers and video game systems. These things typically have high prices,but on this day, there are only a few available at the very【7】(低的) sale prices. So people sometimes fight over these things.Some people will wait in line for hours before【8】(午夜) when stores finally open. People【9】(宿营)out in the cold and sometimes get hurt when the doors open and people rush in.But lately a few stores have been starting sales even【10】(更早地) on Thanksgiving Day. Stores do this to hopefully get more shoppers and stay ahead of the competition.