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People enjoy taking trips. But what are the reasons they leave home? One reason is for (21)________. People travel because they want to broaden their horizons, to learn about other people and other places. They are (22)________about other cultures. When people are tourists, they get a quick look at different ways of (23)________. On a trip, a person can learn directly by visiting(24)________and historic spots. What does a tourist learn when he sees the art museums, visits the (25)________places and other scenic spots in Paris and shops along the River Seine? He gets a (26)________picture, a real life of the French people. He learns about their (27)________: how they feel about business, beauty and history.(28)_____________________________? Does he get the same information that he could get from a book? He might read that Hong Kong is crowded, that there is less than 200 square meters of space for each person,but (29) ________________________will impress him much more. He might read that there are nearly 200 vehicles for every kilometer of roadway, but the sight of so many vehicles parked along the roadside will be a much more vivid lesson.(30)_____________________________: the straight, vertical lines of the tall modern buildings and the moving lines of boats that people live in.
戴维南定理中所指的等效电路是指一个电阻和一个电压源串联的组合，其中电阻等于有源二端网络（ ）。
线性无源二端电阻网络总可以用一个（ ）等效表示。
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