Solution: The function $ S(t) = \left| \sin\left(\frac3\pi7 t\right) + \cos\left(\frac4\pi7 t\right) \right| $ has period $ T $, where both components repeat. The fundamental period is the least common multiple of the periods of $ \sin\left(\frac3\pi7 t\right) $ and $ \cos\left(\frac4\pi7 t\right) $. - Portal da Acústica