题干

如图,AB和CD相交于点O,∠C=∠1,∠D=∠2,求证:∠A=∠B.

证明:∵∠C=∠1,∠D=∠2(已知)

又∵∠1=∠2(____ )

____ (____ )

∴AC∥BD(____ )

____ (两直线平行,内错角相等)

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答案(点此获取答案解析)

对顶角相等,∠C=∠D,等量代换,内错角相等,两直线平行,∠A=∠B

同类题2

根据短文理解,选择正确答案。

    LONDON—Terror struck in the heart of London on Thursday as explosions ripped through three subway trains and blasted(炸开)the roof off a crowded red double-decker bus. At least 37 people were killed and more than 700 wounded in the deadliest attack on the city since the blitz(闪电战)in World War II.

    British Prime Minister Tony Blair blamed Islamic extremists and said the bombings were designed to coincide with the opening in Scotland of a G-8 summit of the world's most powerful leaders. Foreign Secretary Jack Straw said the bombings—which came the day after London won the bid to host the 2012 Olympics--- have the “hallmarks(特点)of an al-Qaida-related attack”.

    Trapped passengers in the Underground railway threw themselves on the floor, some sobbing. As subway cars quickly filled with smoke, people used their umbrellas try to break the windows so that they could get air. Passengers emerged from the Underground covered with blood and soot(黑烟). On the street, in a light rain, buses ferried the wounded, and doctors used a hotel as a hospital.

     “I didn't hear anything, just a flash of light, people screaming, no thoughts of what it was. I just had to get out of the train,” said subway passenger Chris Randall, 28, who was hospitalized with cuts and burns to the face, the legs and hands.

     “It was chaos(混乱),” said Gary Lewis, 32, evacuated(撤离)from a subway train at King's Cross station. “The one haunting(萦绕)image was someone whose face was totally black( with soot) and pouring with blood.”

    Police said there had been no warning and that the blasts at three subway stations went off within 26 minutes, starting at 8:51 a.m. in an Underground train just outside the financial district. Authorities initially blames a power failure but realized it was a terror attack after the bus bombing near the British Museum at 9:47 a.m.—less than an hour after the first explosion.

同类题5

阅读下面现代文,回答问题。

哈勃,太空中的千里眼

      ①古往今来,人们用肉眼观察灿烂的星空,很多发现难免显得粗糙,甚至只是一些臆想。当伽利略等人将望远镜对准了茫茫太空后,人们才逐渐发现了太空的更多奥秘。

      ②在实践中,人们发现用地面的望远镜观测太空,存在两个缺陷:一是地球大气层会吸收红外线和紫外线,二是大气湍流导致地面光学望远镜的角分辨率很难优于1弧秒。1946年,美国天文物理学家斯皮策提出,在太空中安装一台望远镜将突破上述限制。

      ③1990年,这一设想终成现实。美国“发现”号航天飞机将哈勃望远镜送入了太空。然而,哈勃探索宇宙奥秘的征途并非一帆风顺。升空伊始,哈勃就被发现患了“近视眼”,不能辨别140亿光年以外的物体。此后哈勃的陀螺仪、太阳能电池板、影像摄谱仪、绝热毯、冷却剂等都出现过故障,历经五次“手术”才恢复正常。然而哈勃望远镜面临的挑战,也激励着人们推动科研设备及制造技术的革新。每一次维修都让哈勃能够看得更清晰、更遥远。2009年第五次维修后,哈勃望远镜的观测能力已是最初设计的100倍。

      ④哈勃望远镜得到的数据引发了天文学的革命:它确定了宇宙历史为137亿年左右,证实超大质量黑洞处于大多数星系的中心,它让科学家更好地理解恒星和行星的形成,并探测到太阳系外存在有机物。20年来,科学家利用哈勃望远镜的数据发表了7500多篇论文,使之成为历史上最有科学价值的仪器。美国资深宇航员马斯格雷夫说,哈勃望远镜的价值不仅在于它是一台强大的机器,而且在于它把宇宙论、神学、哲学和天文学贯穿了起来。它是人类的一面镜子,让人类自省:这是一个什么样的宇宙?我们处在什么位置?我们是谁,我们该如何作为?

      ⑤虽然哈勃望远镜将于2014年退休,詹姆斯·韦伯太空望远镜将接过它的接力棒,将人类对深层太空的探索继续下去。

                                                                                                                                        (选自《天文爱好者》2011年第6期,略有删改)