for all real numbers

√x˛ =|x|

= x if x≥0

=-x if x≤0

I don't see the error in the line 4 , √1 = 1 since 1>0 already

But the really error is no there

Geezer it is great you found the faulty line but the reasoning is not this,

find the theorem that demonstrates the error in line 2

just to tell you there are other faulty proof for 1=-1 like for example

-1=i.i=√-1 .√-1 =√(-1.-1) = √1 = 1 but where is the fault

this is an other challenge

I'm not telling the answer rapidly [

] , think about it