tan^1

atan stands for arctan     and     arctan is the same as inverse tan.

so

 

tan^(-1)

or

atan      

 

or

you can use the  x²  button in the ribbon and have    tan^-1

or

you can use Latex. Open the Latex box in the ribbon and type  tan^{-1}   and you get      \(tan^{-1}\)