My browser / operating system: Windows NT 10.0, Chrome 78.0.3904.108, Flash 29.0 (release 0)
When I used

([asin v] of (0.5) ::operators)

The answer is 30.000000000000004, but it shuold be 30, because

([sin v] of (30) ::operators)

=0.5
And when I used

set [my variable v] to ([asin v] of (0.5) ::operators)
show variable [my variable v]

, it was 30.
Then I clicked

(my variable)

It said 30.000000000000004 again❗

And this is from the Windows Calculator also said it should be 30
Why?

MapMaths wrote:

My browser / operating system: Windows NT 10.0, Chrome 78.0.3904.108, Flash 29.0 (release 0)
When I used

([asin v] of (0.5) ::operators)

The answer is 30.000000000000004, but it shuold be 30, because

([sin v] of (30) ::operators)

=0.5
And when I used

set [my variable v] to ([asin v] of (0.5) ::operators)
show variable [my variable v]

, it was 30.
Then I clicked

(my variable)

It said 30.000000000000004 again❗

Why?

Well, to compute x from sin(x). arcsin is defined to be the inverse of sin but restricted to a certain range. Hence, arcsin(sin(x))=x if x is within this range (generally either 0 to 2π or −π to π) or a value y such that sin(y)=sin(x) i.e. y=x+2πn or y=π−x+2πm for some n∈Z or m∈Z and y is in this range. Therefore, Asin of 0.5 is 0.523598776 rad. Additionally, In any right triangle, the sine of an angle x is the length of the opposite side (O) divided by the length of the hypotenuse (H). In a formula, it is written as ‘sin’ without the ‘e’: Often remembered as “SOH” - meaning Sine is Opposite over Hypotenuse. So, the sin of 30 should be -0.98803162409. But that's obviously not what you got. Therefore, I'm not sure how you would get those answers for the variable.