从社会效益和经济效益出发,某地投入资金进行生态环境建设,并以此发展旅游产业.根据规划,本年度投入800万元,以后每年投入将比上年减少
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.本年度当地旅游业收入估计为400万元,由于该项建设对旅游业的促进作用,预计今后的旅游业收入每年会比上年增加
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.
(1)设n年内(本年度为第一年)总投入为a
n万元,旅游业总收入为b
n万元.写出a
n,b
n的表达式;
(2)至少经过几年旅游业的总收入才能超过总投入?
考点分析:
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