某城市有学校500所，其中大学10所，中学200所，小学290所，现在选取50所学校进行学生的视力调查，应采取______方法，其中大学取______所，中学取______所，小学取______所．
∵某城市有学校500所，由于学校是由差别明显的层次构成
现在采用分层抽样方法从所有学生中抽取50人．
每个个体被抽到的概率是
=50 500 1 10
∵大学10所，中学200所，小学290所．
∴用分层抽样进行抽样，应该选取大学 1所，中学20所，小学29所
故答案为：分层，1，20，29．
China is a large country of agriculture（农业）.We all know that there are more than 800 million 44 in China. China starts to 45 the science of agriculture much earlier than other countries in the world. While people in Europe and America were trying to shoot wild animals and birds for food, farmers in China were 46 the science of agriculture. For example, Jia Sixie, who lived in the Northern Wei period（386-534 AD）,was one of the farming pioneers. After he finished his work in government, he 47 home and spent his time on research into agriculture. He 48 information, did experiments and learnt from the experience of farmers. He wrote a book called Qi Min Yao Shu. The book 49 has 110000 words is about both farming and gardening. This book is 50 to be an important summary（概要）of the knowledge of farming.
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