You know, it would really be helpful if we could have a peek at
the picture that's "shown below". Just a peek would be enough.
Right now, the only thing I see below is my dog.
Now ... follow me here ... if you're looking for tan(r), then 'r' is
one of the angles in the triangle, and I'm guessing that all three
of those letters are angles.
tan(r) is going to be the ratio of two of the sides ... I mean
(one side) divided by (another side).
There's no way to go any farther, because you haven't given us
any names for the sides, or any way to describe them.
Betcha the names of the sides are on that picture that's supposed
to be shown below.