>用幂级数收敛
>
>0.999999.... = 0.9+0.09+0.009+0.0009.....
>=0.9 + 0.9/10 +0.9/100+0.9/1000......
>=0.9 + 0.9/10 +0.9/10^2+0.9/10^3+0.9/10^4....
>=0.9(1+ (1/10) + (1/10)^2 +(1/10)^3 + (1/10)^4 +...+(1/10)^n)
>
>1+x+x^2+x^3+....+x^n+....当0<x<1时 = 1/(1-x);
>=0.9(1/(1-0.1))
>=0.9(1/0.9) = 1
