I remember in first year of University (Math/Physics) someone saying that arctan(x+y) = (arctan(x) + arctan(y))/(1-arctan(x)arctan(y)). (Basically he thought t

I remember in first year of University (Math/Physics) someone saying that arctan(x+y) = (arctan(x) + arctan(y))/(1-arctan(x)arctan(y)). (Basically he thought that the expansion of arctan(x+y) was just like the expansion of tan(x+y))