How do you evaluate #arctan(1/2)#?

The #arctan(1/2)# è l'angolo #theta# così #tan(theta) = 1/2#

We can imagine such an angle in the first quadrant as being part of a right-triangle with opposite side #=1#, lato adiacente #=2#, and hypotenuse #=sqrt(5)#.

Unfortunately this is not one of the standard angles and the only way (as far as I can tell) to evaluate it is to use a calculator (or something similar) to look-up:

#arctan(1/2) = 0.463648 " radians" = 26.56505^@#