解，设0.9999...=x则10x=9.9999......则，9x=9则x=1或0.999..=9x(0.1+0.01+，，，)=9x0.1x(1-0.1^n)/(1-0.1)=1-0.1^n当n→00则=1
一样大0.999...=1
