The limit problem is as follows:

.

Some thoughts I had about solving this were to express the inverse of tangent as a fraction of cosine divided by sine:

. By doing this I thought that I could then use the

theorem(?) to multiply the numerator by 21 yielding

which equals one. But I don't really know what to do with cosine and this is where I am stuck. The answer in the book is

. If someone could help point me in the right direction I would really appreciate it.

(P.S., sorry if this is in the wrong forum)