The inverse sine and cosine equations mean "angle who's sine/cosine is:" The bounds of the arguments for these functions must be between 1 and -1, the maximum and minimum values of the sine and cosine functions, because no angle can have a sine or cosine that is greater than 1 or smaller than -1.

Also it should be noted that the inverse sine/cosine function is not the same as 1/sine or 1/cosine, although the symbol makes it look like that.