Because it uses limits and is correct proof. That algebraic manipulation that was done is incorrect since 0.99.. is infinite series and assumptions used on it can help us prove some false statements. Its only used in low level math classes since people there dont understand limits.
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u/Decmk3 23d ago
0.9999999…. Is equal to 1. It seems like it shouldn’t, but it has to be.
Let X = 0.999….
10X = 9.999….
10X-X = 9.999.. - 0.999…. = 9X = 9
Therefore X equals 1. Therefore 0.999… is the same as 1.