0.666666.
= 0.6 + 0.06 + 0.006 + 0.0006 + .
= 0.6 + 0.6/10 + 0.006/100 + 0.0006/1000 + .
= lim {0.6×[1 - (1/10)^n]/[1 - 1/10]} [这是等比数列：公比1/10,首项是0.6]
n→∞
= lim {0.6×[1 - (1/10)^n]/[9/10]}
n→∞
= (6/9)lim [1 - (1/10)^n]
n→∞
= (2/3)lim [1 - (1/10)^n]
n→∞
= (2/3)×[1 - 0] [绝对值小于1的数的无穷次幂等于0]
= 2/3