How do you know when to use inverse sin or sin?
when you want to solve this equation to "alpha", then you need the inverse of sin, in this calculator "asin()".
asin( sin( alpha ) ) = asin( opposite/hypotenuse )
alpha = asin( opposite/hypotenuse )
so if "opposite" and a "hypotenuse" of a right triangle is given, you can calculate the angle "alpha".