Dear community, I am trying to manipulate algebraic expressions for the propagation of tidal waves. In particular I would like to expand the trigonometric functions. If I do that as follows, I obtain the expected result.
TrigExpand[Cos[a + b] + Cos[c + d]]
Cos[a] Cos[b] + Cos[c] Cos[d] - Sin[a] Sin[b] - Sin[c] Sin[d]
However, if I try something like
TrigExpand[ArcTan[Cos[a + b] + Cos[c + d]]]
ArcTan[Cos[a + b] + Cos[c + d]]
Mathematica does not seem to recognize it has to expand the trigonometric functions that are in the Argument of ArcTan. How can I force Mathematica to return
ArcTan[Cos[a] Cos[b] + Cos[c] Cos[d] - Sin[a] Sin[b] - Sin[c] Sin[d]]