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例1 某人站在两堵高大的石壁中央,面对一面的石壁用话筒大喊一声,测得从开始大喊至听到第四次回声的总时间为2.5s,则两石壁的间距是( ) (A)283.3 m (B)212.5 m (C)243.0 m (D)340 m 分析和解:人从发声到能听到第四次回声的示意图如图1示.设两石壁间的
Example 1 Someone stood in the middle of two tall stone walls and shouted with a microphone on one side of the stone wall. The total time from the start of shouting to hearing the fourth echo was 2.5s. Then the distance between the two stone walls was () 283.3 m (B) 212.5 m (C) 243.0 m (D) 340 m Analysis and reconciliation: The schematic diagram of a person who hears the fourth echo from vocalization is shown in Fig. 1.