how do I use an inverse trigonometric function

Jboy314 is completely correct.  I just want to stress a point

you use normal trig when you are looking for a length.

You use Inverse trig when you are looking for an angle!

Inverse trig functions use ratios or function values as their inputs and produce angle measures as their outputs versus a regular trig function that does the exact opposite.  When I teach this in geometry i use the phrase "inverses find angles" as a way to remember which is which.

 

As an example, if you wanted to know what measure on the unit circle would produce a y value of 1 you would use sin^-1(1)=pi/2