问题 填空题

A.条件(1)充分,但条件(2)不充分.

B.条件(2)充分,但条件(1)不充分.

C.条件(1)和(2)单独都不充分,但条件(1)和条件(2)联合起来充分.

D.条件(1)充分,条件(2)也充分.

E.条件(1)和(2)单独都不充分,条件(1)和条件(2)联合起来也不充分.

曲线ax2+by2=1通过4个定点.
(1)a+b=1(2)a+b=2().

答案

参考答案:D

解析:

由条件(1),a+b=1,所以ax2+by2=a+b=1,即以a(x2-1)+b(y2-1)=0.对于满足x2=1,y2=1的点(x,y)都在此曲线上,解得x=±1,y=±1.即曲线通过定点(1,1),(1,-1).(-l,1),(-1,-1).所以条件(1)充分.

由条件(2),a+b=2.所以ax2+by2=1可化为对满足的点都在此曲线上,解得x=即曲线过定点

条件(2)也充分.

故本题应选D.

完形填空

The Three Gorges Dam

“Walls of stone to hold back clouds and rain”

Mao Zedong wrote a poem in which he           (梦想) “walls of stone to         (阻止) clouds and rain till a       (光滑的,平坦的) lake       (上升) in the narrow gorges”. Now his dream has           (实现). The        (水力) of the Yangtze River, which is the world’s third longest river, has been           (利用) by the Three Gorges Dam.

The Three Gorges Dam, which is the biggest                 (建筑工程)in China since the building of the Great Wall and the Grand Canal, has been built                  (控制泛滥)and

     hydro-electric power        (为……提供)the central region of China. The dam is nearly 200 metres      (高)and 1.5 kilometres      (宽). It is the largest              (水力发电站)

and dam in the world and has cost more than any other construction project in history.

Sun Yat-sen, who was the leader of the 1911 Revolution, first       (建议,提出)the idea of a dam across the Yangtze River in 1919. Three quarters of China’s energy is         (产生)by burning coal. In 1993, China used 1.2 billion tons of coal for heating and                  (发电). Unfortunately, burning coal      (导致,引起) serious air pollution and increases             (全球变暖). The dam will generate electricity           (相当于)about 40 million tons of coal without causing so much air pollution.

The reservoir has        (淹没)2 cities, 11 counties, 140 towns and more than 4,000 villages. More than a million people who lived in the region have moved from their homes. Now they’re                         (过着幸福的新生活)in different areas.

The Three Gorges area is one of the most beautiful areas of China and the project has flooded some of China’s most famous                 (历史遗迹),      (包括)the Qu Yuan Temple, the Han Watchtower and the Moya Cliff carvings. About 800 historical relics have been      (淹没). Some of them are being          (迁移)and some are being put into museums.

单项选择题