哈哈，就是1
可以看成是一个无穷等比数列0.9,0.09,0.009,0.0009,......的和
0.9999999999999999999......=0.9/(1-0.1)=1
首项为a1，公比为q（|q|<=1)无穷等比数列的和S=a1/(1-q)
设A=0.9999999999999...
则:10A = 9.9999999999999999999
则: 10A-A = 9A = 9
得:A=1
即: 0.999999999999... = 1
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