is there an inverse key

It will help if you know that 

inverse sine = arc sin

$$sin^{-1}(ratio)=asin(ratio)$$

Web2 calc uses both notation.

Otherwise it is the same as any normal hand held calc.

To get inverse of trigonometric functions (sin, cos, tan. .  .) click on "2nd" and they will pop up in place of the regular trig functions. There is also an inverse for numbers, the "1/x" button at the top, fourth from the left.