I actually posted this a while ago and it got answerd by a smart person so here's what I understood
the error is that it assumes that √(x²)=x because if x is negative, it becomes positive that's how the 2nd line is wrong the 2nd line is the first wrong step
it goes from 4-9/2+9/2 so it goes from 4-4.5+4.5 notice the part under the parenthsees √((4-4.5)²) if we evaluate 4-4.5, we get -0.5 but by squaring it we get 0.5
-0.5+4.5=4, yes but when we square it we get 0.5+4.5=5, that is how it works
