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We have to take
√(5-9/2)^2+9/2=-5+9/2+9/2=-5+9=4
Clear??

I was talking about the 3. line. XD
The 3. line is wrong. Maybe a typing error or something. Cause I thing a mistake in the 3. line was not your point. But there is one.

image

Yes this is the trick but if you use √1/4=-1/2 then-1/2+9/2=4
So your view and my view is the same but in different lines

Haha I see.
But the first mistake is the important one. Because = is a relation and you are not allowed to make with wrong arguments new steps from the math point of view.