Radius of turn

[Bell427]

Hay!

I need some help. My instructor gave me a task, to find out, from where the rule of thumb equation for radius of standard turn ( r(nm)= TAS(kt)/200) is derived from. The more precise equation is: r(NM)= TAS(kt)/189,4, for rate R=3°/s turn.

I know that full equation for radius of turn in NM is:
r= TAS(kt)/ rate of turn(1,2,3,)x60x3,14(pi)

thanks for answer!