sin-1?

You just enter -1 in the calculator & press the "inverse sine" key and it should give you an angle of

-90 degrees. Or, if you enter -90 and press the "sine" key, it should give you -1.

I think you might want to know how to access the inverse sine key on the calculator.....just hit the "2nd" key and you will see the  "asin"  key