题目

如图，这是圆桌正上方的灯泡（看作一个点）发出的光线照射到桌面后在地面上形成（圆形）的示意图．已知桌面直径为1.2米，桌面离地面1米．若灯泡离地面3米，则地面上阴影部分的面积为 ㎡． 答案：0.81π ㎡． 【考点】相似三角形的应用． 【分析】如图设C，D分别是桌面和其地面影子的圆心，依题意可以得到△OBC∽△OAD，然后由它们的对应边成比例可以求出地面影子的半径，这样可以求出阴影部分的面积． 【解答】解：如图设C，D分别是桌面和其地面影子的圆心，CB∥AD， ∴△OBC∽△OAD ∴， ∵OD=3Thousands of dead red-winged blackbirds fell from the sky in an Arkansas town on New Year’s Eve after massive injuries, tests by Arkansas officials concluded on Monday. Some 5,000 birds mysteriously fell from the sky in Beebe, Arkansas after dark on New Year’s Eve. “The birds suffered from acute physical injuries leading to internal hemorrhage（大量出血） and death. There was no sign of infectious disease,” the Arkansas Game and Fish Commission said in a statement. The birds were otherwise healthy, it said. One theory is that birds were frightened by New Year's fireworks and flew into buildings or other objects. “Loud noises were reported shortly before the birds began to fall from the sky,” the statement said, adding that blackbirds have poor night vision and seldom fly at night. 　　Another theory was that severe weather such as lightning accounted for the loud noises but this was discounted because the violent weather had already left the area. “We’re leaning more toward a stress event," said Arkansas Game and Fish Commission spokesman Keith Stephens. The commission also is trying to determine what caused the deaths of up to 100,000 fish over a 20-mile stretch of the Arkansas River near a dam in Ozark, 125 miles west of Beebe. The fish were discovered on December 30. Stephens said the commission expected results on the fish tests in probably a month. Since almost all the fish were one species -- bottom-feeding drum, Stephens said, the test was very important. Stephens also said: “The events do not appear related.” Both that section of the river and the air at the site of the bird deaths were tested for toxins(毒素). Beebe is a town of about 4,500 people located 30 miles northeast of the state capital. What is NOT the probable reason why the dead red-winged blackbirds fell from the sky? A. Internal hemorrhage                B. Fireworks C. Lightning weather                          D. Murder In the third paragraph, the underlined word discounted probably mean________. A. not to reduce cost                           B. not to believe C. to make up a part                          D. to explain mystery According to Stephens, we can infer the probable cause of the dead fish is _________.A. cold weather                                  B. poor management   C. disease                                         D. lack of food The passage is mainly developed by __________. A. analyzing causes                           B. making comparisons C. examining differences                    D. following the time order