甲投篮命中率为O．8，乙投篮命中率为0.7，每人投3次，两人恰好都命中2次的概率是多少?
0.169
【错解分析】设“甲恰好投中两次”为事件A，“乙恰好投中两次”为事件B，则两人都恰好投中两次为事件A+B，P(A+B)=P(A)+P(B)： 
本题错误的原因是把相互独立同时发生的事件当成互斥事件来考虑，将两人都恰好投中2次理解为“甲恰好投中两次”与“乙恰好投中两次”的和。
【正解】设“甲恰好投中两次”为事件A，“乙恰好投中两次”为事件B，且A，B相互独立，则两人都恰好投中两次为事件A·B，于是P(A·B)="P(A)×P(B)=" 
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| 阅读理解。 | ||
| A new eight-kilometre road is under construction that links the port area with the motorway system. It is expected to carry 20,000 trucks and cars a day, which greatly reduces the overcrowded traffic in the center of the city. As part of the project, two four-kilometre road tunnels are being built below the central area of the city, one for traffic to the north and the other for traffic to the south. The two tunnels are about 20 metres below the surface and are 12 metres wide, providing for two lanes of traffic in each direction. In the upper part of the tunnel two air-conditioning pipes remove the waste gas of trucks and cars and keep the quality of air inside the tunnel. The lighting is at the top of the tunnel, practically at its highest point. There will also be electronic signs at frequent intervals (间隔). They show traffic conditions ahead, and can be seen clearly by drivers. The wall is made up of four main elements, which include a waterproofing covering and, on the inside of the tunnel, a concrete lining (混凝土衬砌).
Other systems in the tunnel will include emergency phones. | ||
| 1. Each of the tunnels under construction is _____. | ||
| A. eight kilometers long and twenty metres wide B. eight kilometers long and six metres wide C. four kilometers long and twenty metres wide D. four kilometers long and six metres wide | ||
| 2. Which part of the tunnel in the diagram is used to pipe out extra water? | ||
| A. 1. B. 3. C. 5. D. 6. | ||
| 3. Driving in the tunnel, one can know the traffic conditions ahead through _____. | ||
| A. the lights B. the electronic signs C. the trucks and cars D. the emergency phones | ||
| 4. The passage is mainly about _____. | ||
| A. the construction of a road B. the design of a road C. the construction of two tunnels D. the design of two tunnels |
