how do i do inverse tan

you flip over and wait in the sun for as long as you want to get tanned.

with a calc. you need to press 2nd and then the tan key

It will probably help if you know inverse tan is the same as arc tan

$$tan^{-1}\theta = atan\theta$$

It is the same as an ordinary calculator although it you hold youfinger on tan a little longer than usual it will also turn into atan.

so there is two ways of doing it.

Oh and yes - very funny anonymous -I like your humour - thanks. 

Dear Anonymous......I that's called the "Obverse Tan," often denoted in mathematics as Tan^obverse

I don't think there's a calculator button for it (yet), though.....