Formal
Proof of 1 =.999....
The math is pretty straight forward, there is a typo in the last equation of the first page and first equation of the second page, each of the sigmas should be multiplied by a 9.
This is one of the method used in Real analysis to prove such a fact.
one also needs to be aware of the definiton of eqality
x=y iff Abs(x-y) < d for all d>0 in the Set of Real numbers.
Proof,
if x = y , Abs(x-y)=0 , 0< d for all d>0 in the reals so the first half is easy
if abs(x-y) < d for all d> 0 in the reals then x=y
proof
assume x<>y
therefore either x>y or x<y
Case 1. x>y
if x>y then there exists some real number e>0 such that abs(x-y) = e
now if I choose a d = e/2 I have
abs(x-y) = e > e/2 =d so I have
abs(x-y)> d but this violates our given conditon abs(x-y)< d for all d in the reals This is a condratiction therefor our initial assumption is false, x>y cannot be true.
Case 2
x<y
Using the exact same logic as above I can establish a contradiction
since x>y cannot be true and x<y cannot be true it must be that x=y
QED
I have just shared with you a rather simple formal proof of the definition of equality.
Now if you do not agree with this definition you will need to provide a similar definiton so that we know what YOU mean when you say x=y
I have told you what I mean by this statement so there can be no confusion.