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如果向量β可由向量组α1，α2，…线线性表示，则______
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已知A股票的预期报酬率为20%，标准差为40%；B股票的预期报酬率为12%，标准差为13.3%，投资者将25%的资金投资于A股票，75%的资金投资于B股票。[要求]（1）计算投资组合的预期报酬率；（2）若它们的相关系数分别等于1、-1、0和－0.4分别计算投资组合的标准差；（3）对计算结果进行简要说明。
石家庄某专业报关企业，因业务需要该公司需在大连海关办理报关业务，该报关企业只需要在大连海关依法设立分支机构，并向石家庄直属海关办理备案手续，即可在大连海关从事报关服务工作。
One of the most famous bridges in the world is the Golden Gate Bridge that spans San Francisco Bay. Because it was built in 1937, it has become one of the first sights people see if they are coming to San Francisco by ship. The main part of the Golden Gate Bridge is a suspension span. It hangs from steel cables that are supported by two huge towers, one at each end of the bridge. The suspension span was the longest in the world when it was finished in 1937. It measures 4,200 feet between the towers. The floor of thebridge is wide enough for six lanes of cars, three in each direction. There are sidewalks, too, but not many people use them. It costs a lot of time to go on foot from one end of the bridge to the other.The years since 1950 have been called the era of the giant bridge. Most of them have been built in the United States. The Mackinac Bridge in upper Michigan was finished in 1958. It is a suspension bridge, with foundations resting on rock 200 feet below the surface of the water. With a span of 3,800 feet, it was the second in length to the Golden Gate Bridge.In 1964 the longest and heaviest suspension bridge was completed in New York City. Known as the Verrazano--Narrows Bridge, it carries traffic between Staten Island and Brooklyn, parts of the city of New York. Before it was built, commuters to the city had to travel by ferry. Now millions of cars pass over the bridge every year. When a lower deck is added, 48 million cars will be able to cross the bridge in a single year. How many lanes of traffic in the Golden Gate Bridge
能否通过多角化投资来分散，是区分系统风险和非系统风险的主要标志。（）
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