已知fx=x/3x+1数列an满足a1=1 An+1=fan(n属于N*)求证1/an是等差数列

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已知fx=x/3x+1数列an满足a1=1 An+1=fan(n属于N*)求证1/an是等差数列

已知fx=x/3x+1数列an满足a1=1 An+1=fan(n属于N*)求证1/an是等差数列
已知fx=x/3x+1数列an满足a1=1 An+1=fan(n属于N*)求证1/an是等差数列

已知fx=x/3x+1数列an满足a1=1 An+1=fan(n属于N*)求证1/an是等差数列
a(n+1)=f(an)=an/(3an+1)
1/a(n+1)=1/an+3
=> 1/a(n+1)-1/an=3
=> 1/an是公差为3的等差数列

由题有:A(n+1)=f(An)=An/(3An+1)
=> 3A(n+1)An+A(n+1)=An
=> An-A(n+1)=3A(n+1)An (由题可以知道{An}没有零项)
=> 1/A(n+1)-1/An=3
=> 1/An是公差为3的等差数列

a(n+1)=f(an)=an/(3an+1)
1/a(n+1)=1/an+3
得证

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