@pikuma The page seems to have a formatting issue with the formula. They probably meant
atan(√(3) / √(6) / 2) = atan(√(2) / 4).
asin(1 / 3) seems to give the same result.

Edit:
a = √2
b = 4
c = √(a² + b²) = √18
a / c = 1 / 3
atan(a / b) = asin(a / c)