0.999.../Proof by limit manipulation
<
0.999...
Assumptions
Definition from sequences
Term-by-term operations on sequences
The limit of a geometric sequence
Proof
0.999
…
=
lim
n
→
∞
0.
99
…
9
⏟
n
=
lim
n
→
∞
∑
k
=
1
n
9
10
k
=
lim
n
→
∞
(
1
−
1
10
n
)
=
1
−
lim
n
→
∞
1
10
n
=
1.
{\displaystyle 0.999\ldots =\lim _{n\to \infty }0.\underbrace {99\ldots 9} _{n}=\lim _{n\to \infty }\sum _{k=1}^{n}{\frac {9}{10^{k}}}=\lim _{n\to \infty }\left(1-{\frac {1}{10^{n}}}\right)=1-\lim _{n\to \infty }{\frac {1}{10^{n}}}=1.\,}
