背景资料 (高中《数学》第二册 (下 )第 131页例 2 )在一段线路中并联着 3个自动控制的常开关 ,只要其中有一个开关能够闭合 ,线路就能正常工作 .假定在某段时间内每个开关能够闭合的概率都是 0 .7,计算　　　　　图 1在这段时间内线路正常工作的概率 .解　分别记这段时间内开 Background Information (Example 2 of High School Mathematica, Vol. 2 (bottom), Page 131, Example 2) In a section of circuit, three automatic control switches are connected in parallel. As long as one of the two switches can be closed, the circuit will work. The probability that each switch can be closed within a period of time is 0.7, and the probability that the circuit in Figure 1 will work normally during this period is calculated.