已知圆,直线.(1)当为何值时,直线与圆相切.(2)当直线与圆相交于、两点,且时,求直线的方程. 答案:【答案】(1);(2)或.【解析】(1)将圆的方程化为标准形式,得出圆的圆心坐标和半径长,利用圆心到直线的距离等于半径,可计算出实数的值;(2)利用弦长的一半、半径长和弦心距满足勾股定理可求得弦心距,利用点到直线的距离公式可求得实数的值,进而可得出直线的方程.(1)圆的标准方3、 Although man has known asbestos(石棉) for many hundreds of years, it was not until 160 years ago that it
was mined for the first time on the North American continent. H. W. Johns,
owner of a New York City Supply Shop for roofers, was responsible for(对…负责) the opening of that first mine.
Mr. Johns was
given a piece of asbestos which had been found in Italy. He experimented with
the material and then showed its surprising powers to his customers. After
putting on a pair of asbestos gloves, which looked much like ordinary work
gloves, he took red-hot coals from the fireplace and played with them in his
hands. How astonished the customers were to discover that he was not burned at
all. You can well imagine that he had increasing business in asbestos roofing
materials. However, because it was very expensive to transport (carry) them
from Italy to the United States, Mr. Johns sent out a young scientist to seek a
source nearer home. This young man found great vein(岩脉), in the province of Quebec in
Canada.
Ever since 1881
Quebec has led the world in the production of this unusual mineral, which is
made up of magnesium, silicon, iron, and oxygen. When it is mined, the asbestos
is heavy, just as you would expect a mineral to be. When it is separated, a
strange thing happens; the rock breaks down into fine, soft, soapy fibres(纤维).
Scientists do
not know why the rock can be separated easily into threads(线), but they have found thousands of
users of this fireproof material, of the so-called “cloth
of stone”.
59. Which title best expresses the
main idea of this passage?
A. Asbestos mined in
Canada
B. Fireproof matter
C. A “wonder” mineral
D. A new roofing material
60. Johns proved his ability as a
salesman by_______.
A. going into roofing
business B. carrying
asbestos from Italy
C. sending a trained
scientist D.
showing the use of asbestos gloves
61. Which is the most important
character of asbestos that the author wants to show us?
A. It is like
thread.
B. It feels soapy.
C. It burns
easily.
D. It is unusually heavy.
62 The author’s main purpose in writing this
passage was to _______.
A. show the need for more
scientists B. compare asbestos with other minerals
C. increase the sales of
asbestos D. present facts
about asbestos