# Math Help - Harmonic function question

1. ## Harmonic function question

Show that
$4 cos(\alpha t) + 3sin(\alpha t) = 5cos(\alpha t + \phi)$
where $\phi =arc tan(\frac{-3}{4}).$

I believe this is a harmonic function/sinusoid? Any help?

2. Well, I would probably use the addition of angles formula to expand out the RHS.