求极限 x趋近于1+,lim[x/(x-1)-1/lnx]
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求极限 x趋近于1+,lim[x/(x-1)-1/lnx]
先通分
lim(x--->1+)[x/(x-1)-1/lnx]
=lim(x--->1+)[(xlnx-(x-1))/(x-1)lnx]
上下各求导
=lim(x--->1+)[(1+lnx-1)/(lnx+(x-1)/x)]
=lim(x--->1+)[lnx/(lnx+1-1/x)
=lim(x--->1+)[xlnx/(xlnx+x-1)]
再次上下求导
=lim(x--->1+)[(lnx+1)/(lnx+2)]
=(ln1+1)/(ln1+2)
=1/2
lim(x--->1+)[x/(x-1)-1/lnx]
=lim(x--->1+)[(xlnx-(x-1))/(x-1)lnx]
上下各求导
=lim(x--->1+)[(1+lnx-1)/(lnx+(x-1)/x)]
=lim(x--->1+)[lnx/(lnx+1-1/x)
=lim(x--->1+)[xlnx/(xlnx+x-1)]
再次上下求导
=lim(x--->1+)[(lnx+1)/(lnx+2)]
=(ln1+1)/(ln1+2)
=1/2
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