It's not a simplification, it's wrong. Sqrt(square(x)) equals abs(x).

It also equals x with appropriate assumptions (x > 0).

	▲	notarget137 11 minutes ago \| parent \| next [-]
		Well, then sin(x) = x if x is infinitely small
	▲	exe34 43 minutes ago \| parent \| prev [-]
		so there's an unconditionally correct answer (it's also equal to abs(x) for x>0), and then there is an answer that is only correct for half the domain, which requires an additional assumption.