Let's start with the function y=-cos(x).

set x=pi.

therefore, y=-cos(pi)=1.

now lets go backwards.

assume that you have 0.999...=-cos(x)

if 0.999...=1, then 0.999...=1=-cos(x) where x=pi

however, this is not the case, because pi is an irrational and completely predefined number for all of its digits associated with it i.e. 3.14159265358979323846264338327950288419716939937510... ad infinitum.

so setting y=0.999... would yield a value of x that is infinitesimally close to pi but not pi. you might ask which digit of pi is altered to achieve such an infinitesimal aberration from the number one, but it less as elusive as asking where is the missing 0.000... ...01 from the whole number 1 to make 0.999...

the existence of pi is indisputable. each number is as important as the number that precedes it towards infinity, and it is wholly unique. if 0.999... = 1, then you are denying the absolute existence of pi and you should be ashamed of yourselves.

if 0.999... = 1, then 1=2 and 0=3.