limAn/Bn=3,求lim(B1+B2+……+Bn)/(n*A4n)
等差数列An,Bn的公差都不为0,limAn/Bn=3,求lim(B1+B2+……+Bn)/(n*A4n)...
等差数列An,Bn的公差都不为0,limAn/Bn=3,求lim(B1+B2+……+Bn)/(n*A4n)
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2013-12-06
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An=a+(n-1)d1,Bn=b+(n-1)d2
limAn/Bn=lim{[na+n(n-1)d1/2]/[nb+n(n-1)d2/2]}
=lim{[n^2d1/2+na-nd1/2]}/{[n^2d2/2+nb-nd2/2]}
=d1/d2=3
lim(B1+B2+……+Bn)/(n*A4n)
=lim{[n^2d2/2+nb-nd2/2]}/[n{a+(4n-1)d1}]
=lim({[n^2d2/2+nb-nd2/2]}/(4n^2d1+na-nd1)
=lim(n^2d2/4n^2d1)
=d2/4d1=1/12
limAn/Bn=lim{[na+n(n-1)d1/2]/[nb+n(n-1)d2/2]}
=lim{[n^2d1/2+na-nd1/2]}/{[n^2d2/2+nb-nd2/2]}
=d1/d2=3
lim(B1+B2+……+Bn)/(n*A4n)
=lim{[n^2d2/2+nb-nd2/2]}/[n{a+(4n-1)d1}]
=lim({[n^2d2/2+nb-nd2/2]}/(4n^2d1+na-nd1)
=lim(n^2d2/4n^2d1)
=d2/4d1=1/12
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