Solution: Rewrite $ f(x) = \frac\sin^2 x + 4\sin x = \sin x + \frac4\sin x $. Let $ y = \sin x \in (0, 1] $. The function becomes $ f(y) = y + \frac4y $. The derivative $ f'(y) = 1 - \frac4y^2 $ has critical point at $ y = 2 $, but $ y \leq 1 $. Analyze endpoints: as $ y \to 0^+ $, $ f(y) \to \infty $; at $ y = 1 $, $ f(1) = 1 + 4 = 5 $. The minimum is $ 5 $. - Portal da Acústica